Functional Programming for Mortals
Functional Programming for Mortals
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Functional Programming for Mortals

“Love is wise; hatred is foolish. In this world, which is getting more and more closely interconnected, we have to learn to tolerate each other, we have to learn to put up with the fact that some people say things that we don’t like. We can only live together in that way. But if we are to live together, and not die together, we must learn a kind of charity and a kind of tolerance, which is absolutely vital to the continuation of human life on this planet.”

― Bertrand Russell

About This Book

This book is for the typical Scala developer, probably with a Java background, who is both sceptical and curious about the Functional Programming (FP) paradigm. This book justifies every concept with practical examples, including writing a web application.

There are many ways to do Functional Programming in Scala. This book uses scalaz 7.2, the most popular, stable, principled and comprehensive framework. You could instead use the Cats simplified rewrite of scalaz, or roll your own framework.

This book is designed to be read from cover to cover, in the order presented, with a rest between chapters. Earlier chapters encourage coding styles that we will later discredit: similar to how we learn Newton’s theory of gravity as children, and progress to Riemann / Einstein / Maxwell if we become students of physics.

A computer is not necessary to follow along, although we hope that you will gain the confidence to independently study the scalaz source code. Some of the more complex code snippets are available with the book’s source code and those who want practical exercises are encouraged to (re-)implement scalaz (and the example application) using the partial descriptions presented in this book.

We also recommend The Red Book as further reading. It teaches how to write an FP library in Scala from first principles. Try to attend a Fantasyland Institute of Learning training course if you can.

Copyleft Notice

This book is Libre and follows the philosophy of Free Software: you can use this book as you like, the source is available, you can redistribute this book and you can distribute your own version. That means you can print it, photocopy it, e-mail it, upload it to websites, change it, translate it, charge for it, remix it, delete bits, and draw all over it.

This book is Copyleft: if you change the book and distribute your own version, you must also pass these freedoms to its recipients.

This book uses the Creative Commons Attribution ShareAlike 4.0 International (CC BY-SA 4.0) license.

All original code snippets in this book are separately CC0 licensed, you may use them without restriction. Excerpts from scalaz and related libraries maintain their license, reproduced in full in the appendix.

The example application drone-dynamic-agents is distributed under the terms of the GPLv3: only the snippets in this book are available without restriction.


Diego Esteban Alonso Blas, Raúl Raja Martínez and Peter Neyens of 47 degrees, Rúnar Bjarnason, Tony Morris, John de Goes and Edward Kmett for their help explaining the principles of FP. Kenji Yoshida and Jason Zaugg for being the main authors of scalaz, and Paul Chuisano / Miles Sabin for fixing a critical bug in the scala compiler (SI-2712).

Thank you to the readers who gave feedback on early drafts of this text.

Some material was particularly helpful for my own understanding of the concepts that are in this book. Thanks to Juan Manuel Serrano for All Roads Lead to Lambda, Pere Villega for On Free Monads, Dick Wall and Josh Suereth for For: What is it Good For?, Erik Bakker for Options in Futures, how to unsuck them, Noel Markham for ADTs for the Win!, Sukant Hajra for Classy Monad Transformers, Luka Jacobowitz for Optimizing Tagless Final, Vincent Marquez for Index your State, Gabriel Gonzalez for The Continuation Monad, and Yi Lin Wei / Zainab Ali for their tutorials at Hack The Tower meetups.

The helpul souls who patiently explained things to me: Merlin Göttlinger, Edmund Noble, Fabio Labella, Adelbert Chang, Michael Pilquist, Paul Snively, Daniel Spiewak, Stephen Compall, Brian McKenna, Ryan Delucchi, Pedro Rodriguez, Emily Pillmore, Aaron Vargo, Tomas Mikula, Jean-Baptiste Giraudeau, Itamar Ravid, Ross A. Baker and Alexander Konovalov.


If you’d like to set up a project that uses the libraries presented in this book, you will need to use a recent version of Scala with FP-specific features enabled (e.g. in build.sbt):

  scalaVersion in ThisBuild := "2.12.6"
  scalacOptions in ThisBuild ++= Seq(
  libraryDependencies ++= Seq(
    "com.github.mpilquist" %% "simulacrum"     % "0.12.0",
    "org.scalaz"           %% "scalaz-core"    % "7.2.22"
  addCompilerPlugin("org.spire-math" %% "kind-projector" % "0.9.6")
  addCompilerPlugin("org.scalamacros" % "paradise" % "2.1.1" cross CrossVersion.full)

In order to keep our snippets short, we will omit the import section. Unless told otherwise, assume that all snippets have the following imports:

  import scalaz._, Scalaz._
  import simulacrum._

1. Introduction

It is human instinct to be sceptical of a new paradigm. To put some perspective on how far we have come, and the shifts we have already accepted on the JVM, let’s start with a quick recap of the last 20 years.

Java 1.2 introduced the Collections API, allowing us to write methods that abstracted over mutable collections. It was useful for writing general purpose algorithms and was the bedrock of our codebases.

But there was a problem, we had to perform runtime casting:

  public String first(Collection collection) {
    return (String)(collection.get(0));

In response, developers defined domain objects in their business logic that were effectively CollectionOfThings, and the Collection API became implementation detail.

In 2005, Java 5 introduced generics, allowing us to define Collection<Thing>, abstracting over the container and its elements. Generics changed how we wrote Java.

The author of the Java generics compiler, Martin Odersky, then created Scala with a stronger type system, immutable data and multiple inheritance. This brought about a fusion of object oriented (OOP) and functional programming (FP).

For most developers, FP means using immutable data as much as possible, but mutable state is still a necessary evil that must be isolated and managed, e.g. with Akka actors or synchronized classes. This style of FP results in simpler programs that are easier to parallelise and distribute, an improvement over Java. But it is only scratching the surface of the benefits of FP, as we’ll discover in this book.

Scala also brings Future, making it easy to write asynchronous applications. But when a Future makes it into a return type, everything needs to be rewritten to accomodate it, including the tests, which are now subject to arbitrary timeouts.

We have a problem similar to Java 1.0: there is no way of abstracting over execution, much as we had no way of abstracting over collections.

1.1 Abstracting over Execution

Let’s say we want to interact with the user over the command line interface. We can read what the user types and we can write a message to them.

  trait TerminalSync {
    def read(): String
    def write(t: String): Unit
  trait TerminalAsync {
    def read(): Future[String]
    def write(t: String): Future[Unit]

But how do we write generic code that does something as simple as echo the user’s input synchronously or asynchronously depending on our runtime implementation?

We could write a synchronous version and wrap it with Future but now we have to worry about which thread pool we should be using for the work, or we could Await.result on the Future and introduce thread blocking. In either case, it is a lot of boilerplate and we are fundamentally dealing with different APIs that are not unified.

Let’s try to solve the problem like Java 1.2 by introducing a common parent. To do this, we need to use the higher kinded types (HKT) Scala language feature.

We want to define Terminal for a type constructor C[_]. By defining Now to construct to its type parameter (like Id), we can implement a common interface for synchronous and asynchronous terminals:

  trait Terminal[C[_]] {
    def read: C[String]
    def write(t: String): C[Unit]
  type Now[X] = X
  object TerminalSync extends Terminal[Now] {
    def read: String = ???
    def write(t: String): Unit = ???
  object TerminalAsync extends Terminal[Future] {
    def read: Future[String] = ???
    def write(t: String): Future[Unit] = ???

You can think of C as a Context because we say “in the context of executing Now” or “in the Future”.

But we know nothing about C and we can’t do anything with a C[String]. What we need is a kind of execution environment that lets us call a method returning C[T] and then be able to do something with the T, including calling another method on Terminal. We also need a way of wrapping a value as a C[_]. This signature works well:

  trait Execution[C[_]] {
    def doAndThen[A, B](c: C[A])(f: A => C[B]): C[B]
    def create[B](b: B): C[B]

letting us write:

  def echo[C[_]](t: Terminal[C], e: Execution[C]): C[String] =
    e.doAndThen( { in: String =>
      e.doAndThen(t.write(in)) { _: Unit =>

We can now share the echo implementation between synchronous and asynchronous codepaths. We can write a mock implementation of Terminal[Now] and use it in our tests without any timeouts.

Implementations of Execution[Now] and Execution[Future] are reusable by generic methods like echo.

But the code for echo is horrible! Let’s clean it up.

The implicit class Scala language feature gives C some methods. We’ll call these methods flatMap and map for reasons that will become clearer in a moment. Each method takes an implicit Execution[C], but this is nothing more than the flatMap and map that you’re used to on Seq, Option and Future

  object Execution {
    implicit class Ops[A, C[_]](c: C[A]) {
      def flatMap[B](f: A => C[B])(implicit e: Execution[C]): C[B] =
      def map[B](f: A => B)(implicit e: Execution[C]): C[B] =
            e.doAndThen(c)(f andThen e.create)
  def echo[C[_]](implicit t: Terminal[C], e: Execution[C]): C[String] = { in: String =>
      t.write(in).map { _: Unit =>

We can now reveal why we used flatMap as the method name: it lets us use a for comprehension, which is just syntax sugar over nested flatMap and map.

  def echo[C[_]](implicit t: Terminal[C], e: Execution[C]): C[String] =
    for {
      in <-
       _ <- t.write(in)
    } yield in

Our Execution has the same signature as a trait in scalaz called Monad, except doAndThen is flatMap and create is pure. We say that C is monadic when there is an implicit Monad[C] available. In addition, scalaz has the Id type alias.

The takeaway is: if we write methods that operate on monadic types, then we can write sequential code that abstracts over its execution context. Here, we have shown an abstraction over synchronous and asynchronous execution but it can also be for the purpose of more rigorous error handling (where C[_] is Either[Error, _]), managing access to volatile state, performing I/O, or auditing of the session.

1.2 Pure Functional Programming

Functional Programming is the act of writing programs with pure functions. Pure functions have three properties:

  • Total: return a value for every possible input
  • Deterministic: return the same value for the same input
  • Inculpable: no (direct) interaction with the world or program state.

Together, these properties give us an unprecedented ability to reason about our code. For example, input validation is easier to isolate with totality, caching is possible when functions are deterministic, and interacting with the world is easier to control, and test, when functions are inculpable.

The kinds of things that break these properties are side effects: directly accessing or changing mutable state (e.g. maintaining a var in a class or using a legacy API that is impure), communicating with external resources (e.g. files or network lookup), or throwing exceptions.

We write pure functions by avoiding exceptions, and interacting with the world only through a safe F[_] execution context.

In the previous section, we abstracted over execution and defined echo[Id] and echo[Future]. We might reasonably expect that calling any echo will not perform any side effects, because it is pure. However, if we use Future or Id as the execution context, our application will start listening to stdin:

  val futureEcho: Future[String] = echo[Future]

We have broken purity and are no longer writing FP code: futureEcho is the result of running echo once. Future conflates the definition of a program with interpreting it (running it). As a result, applications built with Future are difficult to reason about.

We can define a simple safe F[_] execution context

  class IO[A](val interpret: () => A) {
    def map[B](f: A => B): IO[B] = IO(f(interpret()))
    def flatMap[B](f: A => IO[B]): IO[B] = IO(f(interpret()).interpret())
  object IO {
    def apply[A](a: =>A): IO[A] = new IO(() => a)

which lazily evaluates a thunk. IO is just a data structure that references (potentially) impure code, it isn’t actually running anything. We can implement Terminal[IO]

  object TerminalIO extends Terminal[IO] {
    def read: IO[String]           = IO { io.StdIn.readLine }
    def write(t: String): IO[Unit] = IO { println(t) }

and call echo[IO] to get back a value

  val delayed: IO[String] = echo[IO]

This val delayed can be reused, it is just the definition of the work to be done. We can map the String and compose additional programs, much as we would map over a Future. IO keeps us honest that we are depending on some interaction with the world, but does not prevent us from accessing the output of that interaction.

The impure code inside the IO is only evaluated when we .interpret() the value, which is an impure action


An application composed of IO programs is only interpreted once, in the main method, which is also called the end of the world.

In this book, we expand on the concepts introduced in this chapter and show how to write maintainable, pure functions, that achieve your business’s objectives.

2. For Comprehensions

Scala’s for comprehension is the ideal FP abstraction for sequential programs that interact with the world. Since we’ll be using it a lot, we’re going to relearn the principles of for and how scalaz can help us to write cleaner code.

This chapter doesn’t try to write pure programs and the techniques are applicable to non-FP codebases.

2.1 Syntax Sugar

Scala’s for is just a simple rewrite rule, also called syntax sugar, that doesn’t have any contextual information.

To see what a for comprehension is doing, we use the show and reify feature in the REPL to print out what code looks like after type inference.

  scala> import scala.reflect.runtime.universe._
  scala> val a, b, c = Option(1)
  scala> show { reify {
           for { i <- a ; j <- b ; k <- c } yield (i + j + k)
         } }
    ((i) => $read.b.flatMap(
      ((j) => $
        ((k) => i.$plus(j).$plus(k)))))))

There is a lot of noise due to additional sugarings (e.g. + is rewritten $plus, etc). We’ll skip the show and reify for brevity when the REPL line is reify>, and manually clean up the generated code so that it doesn’t become a distraction.

  reify> for { i <- a ; j <- b ; k <- c } yield (i + j + k)
  a.flatMap {
    i => b.flatMap {
      j => {
        k => i + j + k }}}

The rule of thumb is that every <- (called a generator) is a nested flatMap call, with the final generator a map containing the yield body.

2.1.1 Assignment

We can assign values inline like ij = i + j (a val keyword is not needed).

  reify> for {
           i <- a
           j <- b
           ij = i + j
           k <- c
         } yield (ij + k)
  a.flatMap {
    i => { j => (j, i + j) }.flatMap {
      case (j, ij) => {
        k => ij + k }}}

A map over the b introduces the ij which is flat-mapped along with the j, then the final map for the code in the yield.

Unfortunately we cannot assign before any generators. It has been requested as a language feature but has not been implemented:

  scala> for {
           initial = getDefault
           i <- a
         } yield initial + i
  <console>:1: error: '<-' expected but '=' found.

We can workaround the limitation by defining a val outside the for

  scala> val initial = getDefault
  scala> for { i <- a } yield initial + i

or create an Option out of the initial assignment

  scala> for {
           initial <- Option(getDefault)
           i <- a
         } yield initial + i

2.1.2 Filter

It is possible to put if statements after a generator to filter values by a predicate

  reify> for {
           i  <- a
           j  <- b
           if i > j
           k  <- c
         } yield (i + j + k)
  a.flatMap {
    i => b.withFilter {
      j => i > j }.flatMap {
        j => {
          k => i + j + k }}}

Older versions of scala used filter, but Traversable.filter creates new collections for every predicate, so withFilter was introduced as the more performant alternative.

We can accidentally trigger a withFilter by providing type information: it is actually interpreted as a pattern match.

  reify> for { i: Int <- a } yield i
  a.withFilter {
    case i: Int => true
    case _      => false
  }.map { case i: Int => i }

Like in assignment, a generator can use a pattern match on the left hand side. But unlike assignment (which throws MatchError on failure), generators are filtered and will not fail at runtime. However, there is an inefficient double application of the pattern.

2.1.3 For Each

Finally, if there is no yield, the compiler will use foreach instead of flatMap, which is only useful for side-effects.

  reify> for { i <- a ; j <- b } println(s"$i $j")
  a.foreach { i => b.foreach { j => println(s"$i $j") } }

2.1.4 Summary

The full set of methods supported by for comprehensions do not share a common super type; each generated snippet is independently compiled. If there were a trait, it would roughly look like:

  trait ForComprehensible[C[_]] {
    def map[A, B](f: A => B): C[B]
    def flatMap[A, B](f: A => C[B]): C[B]
    def withFilter[A](p: A => Boolean): C[A]
    def foreach[A](f: A => Unit): Unit

If the context (C[_]) of a for comprehension doesn’t provide its own map and flatMap, all is not lost. If an implicit scalaz.Bind[T] is available for T, it will provide map and flatMap.

2.2 Unhappy path

So far we’ve only looked at the rewrite rules, not what is happening in map and flatMap. Let’s consider what happens when the for context decides that it can’t proceed any further.

In the Option example, the yield is only called when i,j,k are all defined.

  for {
    i <- a
    j <- b
    k <- c
  } yield (i + j + k)

If any of a,b,c are None, the comprehension short-circuits with None but it doesn’t tell us what went wrong.

If we use Either, then a Left will cause the for comprehension to short circuit with extra information, much better than Option for error reporting:

  scala> val a = Right(1)
  scala> val b = Right(2)
  scala> val c: Either[String, Int] = Left("sorry, no c")
  scala> for { i <- a ; j <- b ; k <- c } yield (i + j + k)
  Left(sorry, no c)

And lastly, let’s see what happens with a Future that fails:

  scala> import scala.concurrent._
  scala> import
  scala> for {
           i <- Future.failed[Int](new Throwable)
           j <- Future { println("hello") ; 1 }
         } yield (i + j)
  scala> Await.result(f, duration.Duration.Inf)
  caught java.lang.Throwable

The Future that prints to the terminal is never called because, like Option and Either, the for comprehension short circuits.

Short circuiting for the unhappy path is a common and important theme. for comprehensions cannot express resource cleanup: there is no way to try / finally. This is good, in FP it puts a clear ownership of responsibility for unexpected error recovery and resource cleanup onto the context (which is usually a Monad as we’ll see later), not the business logic.

2.3 Gymnastics

Although it is easy to rewrite simple sequential code as a for comprehension, sometimes we’ll want to do something that appears to require mental summersaults. This section collects some practical examples and how to deal with them.

2.3.1 Fallback Logic

Let’s say we are calling out to a method that returns an Option and if it is not successful we want to fallback to another method (and so on and so on), like when we’re using a cache:

  def getFromRedis(s: String): Option[String]
  def getFromSql(s: String): Option[String]
  getFromRedis(key) orElse getFromSql(key)

If we have to do this for an asynchronous version of the same API

  def getFromRedis(s: String): Future[Option[String]]
  def getFromSql(s: String): Future[Option[String]]

then we have to be careful not to do extra work because

  for {
    cache <- getFromRedis(key)
    sql   <- getFromSql(key)
  } yield cache orElse sql

will run both queries. We can pattern match on the first result but the type is wrong

  for {
    cache <- getFromRedis(key)
    res   <- cache match {
               case Some(_) => cache !!! wrong type !!!
               case None    => getFromSql(key)
  } yield res

We need to create a Future from the cache

  for {
    cache <- getFromRedis(key)
    res   <- cache match {
               case Some(_) => Future.successful(cache)
               case None    => getFromSql(key)
  } yield res

Future.successful creates a new Future, much like an Option or List constructor.

If functional programming was like this all the time, it’d be a nightmare. Thankfully these tricky situations are the corner cases.

2.3.2 Early Exit

Let’s say we have some condition that should exit early with a successful value.

If we want to exit early with an error, it is standard practice in OOP to throw an exception

  def getA: Int = ...
  val a = getA
  require(a > 0, s"$a must be positive")
  a * 10

which can be rewritten async

  def getA: Future[Int] = ...
  def error(msg: String): Future[Nothing] =
    Future.failed(new RuntimeException(msg))
  for {
    a <- getA
    b <- if (a <= 0) error(s"$a must be positive")
         else Future.successful(a)
  } yield b * 10

But if we want to exit early with a successful return value, the simple synchronous code:

  def getB: Int = ...
  val a = getA
  if (a <= 0) 0
  else a * getB

translates into a nested for comprehension when our dependencies are asynchronous:

  def getB: Future[Int] = ...
  for {
    a <- getA
    c <- if (a <= 0) Future.successful(0)
         else for { b <- getB } yield a * b
  } yield c

2.4 Incomprehensible

The context we’re comprehending over must stay the same: we can’t mix contexts.

  scala> def option: Option[Int] = ...
  scala> def future: Future[Int] = ...
  scala> for {
           a <- option
           b <- future
         } yield a * b
  <console>:23: error: type mismatch;
   found   : Future[Int]
   required: Option[?]
           b <- future

Nothing can help us mix arbitrary contexts in a for comprehension because the meaning is not well defined.

But when we have nested contexts the intention is usually obvious yet the compiler still doesn’t accept our code.

  scala> def getA: Future[Option[Int]] = ...
  scala> def getB: Future[Option[Int]] = ...
  scala> for {
           a <- getA
           b <- getB
         } yield a * b
  <console>:30: error: value * is not a member of Option[Int]
         } yield a * b

Here we want for to take care of the outer context and let us write our code on the inner Option. Hiding the outer context is exactly what a monad transformer does, and scalaz provides implementations for Option and Either named OptionT and EitherT respectively.

The outer context can be anything that normally works in a for comprehension, but it needs to stay the same throughout.

We create an OptionT from each method call. This changes the context of the for from Future[Option[_]] to OptionT[Future, _].

  scala> val result = for {
           a <- OptionT(getA)
           b <- OptionT(getB)
         } yield a * b
  result: OptionT[Future, Int] = OptionT(Future(<not completed>))

.run returns us to the original context

  res: Future[Option[Int]] = Future(<not completed>)

Alternatively, OptionT[Future, Int] has getOrElse and getOrElseF methods, taking Int and Future[Int] respectively, returning a Future[Int].

The monad transformer also allows us to mix Future[Option[_]] calls with methods that just return plain Future via .liftM[OptionT] (provided by scalaz):

  scala> def getC: Future[Int] = ...
  scala> val result = for {
           a <- OptionT(getA)
           b <- OptionT(getB)
           c <- getC.liftM[OptionT]
         } yield a * b / c
  result: OptionT[Future, Int] = OptionT(Future(<not completed>))

and we can mix with methods that return plain Option by wrapping them in Future.successful (.pure[Future]) followed by OptionT

  scala> def getD: Option[Int] = ...
  scala> val result = for {
           a <- OptionT(getA)
           b <- OptionT(getB)
           c <- getC.liftM[OptionT]
           d <- OptionT(getD.pure[Future])
         } yield (a * b) / (c * d)
  result: OptionT[Future, Int] = OptionT(Future(<not completed>))

It is messy again, but it is better than writing nested flatMap and map by hand. We can clean it up with a DSL that handles all the required conversions into OptionT[Future, _]

  def liftFutureOption[A](f: Future[Option[A]]) = OptionT(f)
  def liftFuture[A](f: Future[A]) = f.liftM[OptionT]
  def liftOption[A](o: Option[A]) = OptionT(o.pure[Future])
  def lift[A](a: A)               = liftOption(Option(a))

combined with the |> operator, which applies the function on the right to the value on the left, to visually separate the logic from the transformers

  scala> val result = for {
           a <- getA       |> liftFutureOption
           b <- getB       |> liftFutureOption
           c <- getC       |> liftFuture
           d <- getD       |> liftOption
           e <- 10         |> lift
         } yield e * (a * b) / (c * d)
  result: OptionT[Future, Int] = OptionT(Future(<not completed>))

This approach also works for EitherT (and others) as the inner context, but their lifting methods are more complex and require parameters. Scalaz provides monad transformers for a lot of its own types, so it is worth checking if one is available.

Implementing a monad transformer is an advanced topic. Although ListT exists, it should be avoided because it can unintentionally reorder flatMap calls according to A better alternative is StreamT, which we will visit later.

3. Application Design

In this chapter we will write the business logic and tests for a purely functional server application.

3.1 Specification

Our application will manage a just-in-time build farm on a shoestring budget. It will listen to a Drone Continuous Integration server, and spawn worker agents using Google Container Engine (GKE) to meet the demand of the work queue.

Drone receives work when a contributor submits a github pull request to a managed project. Drone assigns the work to its agents, each processing one job at a time.

The goal of our app is to ensure that there are enough agents to complete the work, with a cap on the number of agents, whilst minimising the total cost. Our app needs to know the number of items in the backlog and the number of available agents.

Google can spawn nodes, each can host multiple drone agents. When an agent starts up, it registers itself with drone and drone takes care of the lifecycle (including keep-alive calls to detect removed agents).

GKE charges a fee per minute of uptime, rounded up to the nearest hour for each node. One does not simply spawn a new node for each job in the work queue, we must re-use nodes and retain them until their 59th minute to get the most value for money.

Our app needs to be able to start and stop nodes, as well as check their status (e.g. uptimes, list of inactive nodes) and to know what time GKE believes it to be.

In addition, there is no API to talk directly to an agent so we do not know if any individual agent is performing any work for the drone server. If we accidentally stop an agent whilst it is performing work, it is inconvenient and requires a human to restart the job.

Contributors can manually add agents to the farm, so counting agents and nodes is not equivalent. We don’t need to supply any nodes if there are agents available.

The failure mode should always be to take the least costly option.

Both Drone and GKE have a JSON over REST API with OAuth 2.0 authentication.

3.2 Interfaces / Algebras

Let’s codify the architecture diagram from the previous section.

In FP, an algebra takes the place of an interface in Java, or the set of valid messages for an Actor in Akka. This is the layer where we define all side-effecting interactions of our system.

There is tight iteration between writing the business logic and the algebra: it is a good level of abstraction to design a system.

  package algebra
  import java.time.Instant
  import scalaz.NonEmptyList
  trait Drone[F[_]] {
    def getBacklog: F[Int]
    def getAgents: F[Int]
  final case class MachineNode(id: String)
  trait Machines[F[_]] {
    def getTime: F[Instant]
    def getManaged: F[NonEmptyList[MachineNode]]
    def getAlive: F[Map[MachineNode, Instant]]
    def start(node: MachineNode): F[MachineNode]
    def stop(node: MachineNode): F[MachineNode]

We’ve used NonEmptyList, easily created by calling .toNel on the stdlib’s List (returning an Option[NonEmptyList]), otherwise everything should be familiar.

3.3 Business Logic

Now we write the business logic that defines the application’s behaviour, considering only the happy path.

First, the imports

  package logic
  import java.time.Instant
  import java.time.temporal.ChronoUnit
  import scala.concurrent.duration._
  import scalaz._
  import Scalaz._
  import algebra._

We need a WorldView class to hold a snapshot of our knowledge of the world. If we were designing this application in Akka, WorldView would probably be a var in a stateful Actor.

WorldView aggregates the return values of all the methods in the algebras, and adds a pending field to track unfulfilled requests.

  final case class WorldView(
    backlog: Int,
    agents: Int,
    managed: NonEmptyList[MachineNode],
    alive: Map[MachineNode, Instant],
    pending: Map[MachineNode, Instant],
    time: Instant

Now we are ready to write our business logic, but we need to indicate that we depend on Drone and Machines.

We create a module to contain our main business logic. A module is pure and depends only on other modules, algebras and pure functions.

  final class DynAgents[F[_]](implicit
                              M: Monad[F],
                              d: Drone[F],
                              m: Machines[F]) {

The implicit Monad[F] means that F is monadic, allowing us to use map, pure and, of course, flatMap via for comprehensions.

We have access to the algebra of Drone and Machines as d and m, respectively. Declaring injected dependencies this way should be familiar if you’ve ever used Spring’s @Autowired.

Our business logic will run in an infinite loop (pseudocode)

  state = initial()
  while True:
    state = update(state)
    state = act(state)

We must write three functions: initial, update and act, all returning an F[WorldView].

3.3.1 initial

In initial we call all external services and aggregate their results into a WorldView. We default the pending field to an empty Map.

  def initial: F[WorldView] = for {
    db <- d.getBacklog
    da <- d.getAgents
    mm <- m.getManaged
    ma <- m.getAlive
    mt <- m.getTime
  } yield WorldView(db, da, mm, ma, Map.empty, mt)

Recall from Chapter 1 that flatMap (i.e. when we use the <- generator) allows us to operate on a value that is computed at runtime. When we return an F[_] we are returning another program to be interpreted at runtime, that we can then flatMap. This is how we safely chain together sequential side-effecting code, whilst being able to provide a pure implementation for tests. FP could be described as Extreme Mocking.

3.3.2 update

update should call initial to refresh our world view, preserving known pending actions.

If a node has changed state, we remove it from pending and if a pending action is taking longer than 10 minutes to do anything, we assume that it failed and forget that we asked to do it.

  def update(old: WorldView): F[WorldView] = for {
    snap <- initial
    changed = symdiff(old.alive.keySet, snap.alive.keySet)
    pending = (old.pending -- changed).filterNot {
      case (_, started) => timediff(started, snap.time) >= 10.minutes
    update = snap.copy(pending = pending)
  } yield update
  private def symdiff[T](a: Set[T], b: Set[T]): Set[T] =
    (a union b) -- (a intersect b)
  private def timediff(from: Instant, to: Instant): FiniteDuration =
    ChronoUnit.MINUTES.between(from, to).minutes

Note that we use assignment for pure functions like symdiff, timediff and copy. Pure functions don’t need test mocks, they have explicit inputs and outputs, so you could move all pure code into standalone methods on a stateless object, testable in isolation. We’re happy testing only the public methods, preferring that our business logic is easy to read.

3.3.3 act

The act method is slightly more complex, so we’ll split it into two parts for clarity: detection of when an action needs to be taken, followed by taking action. This simplification means that we can only perform one action per invocation, but that is reasonable because we can control the invocations and may choose to re-run act until no further action is taken.

We write the scenario detectors as extractors for WorldView, which is nothing more than an expressive way of writing if / else conditions.

We need to add agents to the farm if there is a backlog of work, we have no agents, we have no nodes alive, and there are no pending actions. We return a candidate node that we would like to start:

  private object NeedsAgent {
    def unapply(world: WorldView): Option[MachineNode] = world match {
      case WorldView(backlog, 0, managed, alive, pending, _)
           if backlog > 0 && alive.isEmpty && pending.isEmpty
             => Option(managed.head)
      case _ => None

If there is no backlog, we should stop all nodes that have become stale (they are not doing any work). However, since Google charge per hour we only shut down machines in their 58th+ minute to get the most out of our money. We return the non-empty list of nodes to stop.

As a financial safety net, all nodes should have a maximum lifetime of 5 hours.

  private object Stale {
    def unapply(world: WorldView): Option[NonEmptyList[MachineNode]] =
      world match {
        case WorldView(backlog, _, _, alive, pending, time) if alive.nonEmpty =>
          (alive -- pending.keys).collect {
            case (n, started)
                if backlog == 0 && timediff(started, time).toMinutes % 60 >= 58 =>
            case (n, started) if timediff(started, time) >= 5.hours => n
        case _ => None

Now that we have detected the scenarios that can occur, we can write the act method. When we schedule a node to be started or stopped, we add it to pending noting the time that we scheduled the action.

  def act(world: WorldView): F[WorldView] = world match {
    case NeedsAgent(node) =>
      for {
        _ <- m.start(node)
        update = world.copy(pending = Map(node -> world.time))
      } yield update
    case Stale(nodes) =>
      nodes.foldLeftM(world) { (world, n) =>
        for {
          _ <- m.stop(n)
          update = world.copy(pending = world.pending + (n -> world.time))
        } yield update
    case _ => world.pure[F]

Because NeedsAgent and Stale do not cover all possible situations, we need a catch-all case _ to do nothing. Recall from Chapter 2 that .pure creates the for’s (monadic) context from a value.

foldLeftM is like foldLeft over nodes, but each iteration of the fold may return a monadic value. In our case, each iteration of the fold returns F[WorldView].

The M is for Monadic and you will find more of these lifted methods that behave as one would expect, taking monadic values in place of values.

3.4 Unit Tests

The FP approach to writing applications is a designer’s dream: you can delegate writing the implementations of algebras to your team members while focusing on making your business logic meet the requirements.

Our application is highly dependent on timing and third party webservices. If this was a traditional OOP application, we’d create mocks for all the method calls, or test actors for the outgoing mailboxes. FP mocking is equivalent to providing an alternative implementation of dependency algebras. The algebras already isolate the parts of the system that need to be mocked — everything else is pure.

We’ll start with some test data

  object Data {
    val node1   = MachineNode("1243d1af-828f-4ba3-9fc0-a19d86852b5a")
    val node2   = MachineNode("550c4943-229e-47b0-b6be-3d686c5f013f")
    val managed = NonEmptyList(node1, node2)
    import Instant.parse
    val time1 = parse("2017-03-03T18:07:00.000+01:00[Europe/London]")
    val time2 = parse("2017-03-03T18:59:00.000+01:00[Europe/London]") // +52 mins
    val time3 = parse("2017-03-03T19:06:00.000+01:00[Europe/London]") // +59 mins
    val time4 = parse("2017-03-03T23:07:00.000+01:00[Europe/London]") // +5 hours
    val needsAgents = WorldView(5, 0, managed, Map.empty, Map.empty, time1)
  import Data._

We implement algebras by extending Drone and Machines with a specific monadic context, Id being the simplest.

Our “mock” implementations simply play back a fixed WorldView. We’ve isolated the state of our system, so we can use var to store the state:

  class Mutable(state: WorldView) {
    var started, stopped: Int = 0
    implicit val drone: Drone[Id] = new Drone[Id] {
      def getBacklog: Int = state.backlog
      def getAgents: Int = state.agents
    implicit val machines: Machines[Id] = new Machines[Id] {
      def getAlive: Map[MachineNode, Instant] = state.alive
      def getManaged: NonEmptyList[MachineNode] = state.managed
      def getTime: Instant = state.time
      def start(node: MachineNode): MachineNode = { started += 1 ; node }
      def stop(node: MachineNode): MachineNode = { stopped += 1 ; node }
    val program = new DynAgents[Id]

When we write a unit test (here using FlatSpec from scalatest), we create an instance of Mutable and then import all of its members.

Our implicit drone and machines both use the Id execution context and therefore interpreting this program with them returns an Id[WorldView] that we can assert on.

In this trivial case we just check that the initial method returns the same value that we use in the static implementations:

  "Business Logic" should "generate an initial world view" in {
    val mutable = new Mutable(needsAgents)
    import mutable._
    program.initial shouldBe needsAgents

We can create more advanced tests of the update and act methods, helping us flush out bugs and refine the requirements:

  it should "remove changed nodes from pending" in {
    val world = WorldView(0, 0, managed, Map(node1 -> time3), Map.empty, time3)
    val mutable = new Mutable(world)
    import mutable._
    val old = world.copy(alive = Map.empty,
                         pending = Map(node1 -> time2),
                         time = time2)
    program.update(old) shouldBe world
  it should "request agents when needed" in {
    val mutable = new Mutable(needsAgents)
    import mutable._
    val expected = needsAgents.copy(
      pending = Map(node1 -> time1)
    program.act(needsAgents) shouldBe expected
    mutable.stopped shouldBe 0
    mutable.started shouldBe 1

It would be boring to go through the full test suite. Convince yourself with a thought experiment that the following tests are easy to implement using the same approach:

  • not request agents when pending
  • don’t shut down agents if nodes are too young
  • shut down agents when there is no backlog and nodes will shortly incur new costs
  • not shut down agents if there are pending actions
  • shut down agents when there is no backlog if they are too old
  • shut down agents, even if they are potentially doing work, if they are too old
  • ignore unresponsive pending actions during update

All of these tests are synchronous and isolated to the test runner’s thread (which could be running tests in parallel). If we’d designed our test suite in Akka, our tests would be subject to arbitrary timeouts and failures would be hidden in logfiles.

The productivity boost of simple tests for business logic cannot be overstated. Consider that 90% of an application developer’s time interacting with the customer is in refining, updating and fixing these business rules. Everything else is implementation detail.

3.5 Parallel

The application that we have designed runs each of its algebraic methods sequentially. But there are some obvious places where work can be performed in parallel.

3.5.1 initial

In our definition of initial we could ask for all the information we need at the same time instead of one query at a time.

As opposed to flatMap for sequential operations, scalaz uses Apply syntax for parallel operations:

  ^^^^(d.getBacklog, d.getAgents, m.getManaged, m.getAlive, m.getTime)

which can also use infix notation:

  (d.getBacklog |@| d.getAgents |@| m.getManaged |@| m.getAlive |@| m.getTime)

If each of the parallel operations returns a value in the same monadic context, we can apply a function to the results when they all return. Rewriting update to take advantage of this:

  def initial: F[WorldView] =
    ^^^^(d.getBacklog, d.getAgents, m.getManaged, m.getAlive, m.getTime) {
      case (db, da, mm, ma, mt) => WorldView(db, da, mm, ma, Map.empty, mt)

3.5.2 act

In the current logic for act, we are stopping each node sequentially, waiting for the result, and then proceeding. But we could stop all the nodes in parallel and then update our view of the world.

A disadvantage of doing it this way is that any failures will cause us to short-circuit before updating the pending field. But that’s a reasonable tradeoff since our update will gracefully handle the case where a node is shut down unexpectedly.

We need a method that operates on NonEmptyList that allows us to map each element into an F[MachineNode], returning an F[NonEmptyList[MachineNode]]. The method is called traverse, and when we flatMap over it we get a NonEmptyList[MachineNode] that we can deal with in a simple way:

  for {
    stopped <- nodes.traverse(m.stop)
    updates = -> world.time).toList.toMap
    update = world.copy(pending = world.pending ++ updates)
  } yield update

Arguably, this is easier to understand than the sequential version.

3.5.3 Parallel Interpretation

Marking something as suitable for parallel execution does not guarantee that it will be executed in parallel: that is the responsibility of the implementation. Not to state the obvious: parallel execution is supported by Future, but not Id.

Of course, we need to be careful when implementing algebras such that they can perform operations safely in parallel, perhaps requiring protecting internal state with concurrency locks or actors.

3.6 Summary

  1. algebras define the interface between systems.
  2. modules define pure logic and depend on algebras and other modules.
  3. Test implementations can mock out the side-effecting parts of the system, enabling a high level of test coverage for the business logic.
  4. algebraic methods can be performed in parallel by taking their product or traversing sequences (caveat emptor, revisited later).

4. Data and Functionality

From OOP we are used to thinking about data and functionality together: class hierarchies carry methods, and traits can demand that data fields exist. Runtime polymorphism of an object is in terms of “is a” relationships, requiring classes to inherit from common interfaces. This can get messy as a codebase grows. Simple data types become obscured by hundreds of lines of methods, trait mixins suffer from initialisation order errors, and testing / mocking of highly coupled components becomes a chore.

FP takes a different approach, defining data and functionality separately. In this chapter, we will cover the basics of data types and the advantages of constraining ourselves to a subset of the Scala language. We will also discover typeclasses as a way to achieve compiletime polymorphism: thinking about functionality of a data structure in terms of “has a” rather than “is a” relationships.

4.1 Data

The fundamental building blocks of data types are

  • final case class also known as products
  • sealed abstract class also known as coproducts
  • case object and Int, Double, String (etc) values

with no methods or fields other than the constructor parameters.

The collective name for products, coproducts and values is Algebraic Data Type (ADT).

We compose data types from the AND and XOR (exclusive OR) Boolean algebra: a product contains every type that it is composed of, but a coproduct can be only one. For example

  • product: ABC = a AND b AND c
  • coproduct: XYZ = x XOR y XOR z

written in Scala

  // values
  case object A
  type B = String
  type C = Int
  // product
  final case class ABC(a: A.type, b: B, c: C)
  // coproduct
  sealed abstract class XYZ
  case object X extends XYZ
  case object Y extends XYZ
  final case class Z(b: B) extends XYZ

4.1.1 Generalised ADTs

When we introduce a type parameter into an ADT, we call it a Generalised Algebraic Data Type (GADT).

scalaz.IList, a safe alternative to the stdlib List, is a GADT:

  sealed abstract class IList[A]
  final case class INil[A]() extends IList[A]
  final case class ICons[A](head: A, tail: IList[A]) extends IList[A]

If an ADT refers to itself, we call it a recursive type. IList is recursive because ICons contains a reference to IList.

4.1.2 Functions on ADTs

ADTs can contain pure functions

  final case class UserConfiguration(accepts: Int => Boolean)

But ADTs that contain functions come with some caveats as they don’t translate perfectly onto the JVM. For example, legacy Serializable, hashCode, equals and toString do not behave as one might reasonably expect.

Unfortunately, Serializable is used by popular frameworks, despite far superior alternatives. A common pitfall is forgetting that Serializable may attempt to serialise the entire closure of a function, which can crash production servers. A similar caveat applies to legacy Java classes such as Throwable, which can carry references to arbitrary objects. This is one of the reasons why we restrict what can live on an ADT.

We will explore alternatives to the legacy methods when we discuss the scalaz library in the next chapter, at the cost of losing interoperability with some legacy Java and Scala code.

4.1.3 Exhaustivity

It is important that we use sealed abstract class, not just abstract class, when defining a data type. Sealing a class means that all subtypes must be defined in the same file, allowing the compiler to know about them in pattern match exhaustivity checks and in macros that eliminate boilerplate. e.g.

  scala> sealed abstract class Foo
         final case class Bar(flag: Boolean) extends Foo
         final case object Baz extends Foo
  scala> def thing(foo: Foo) = foo match {
           case Bar(_) => true
  <console>:14: error: match may not be exhaustive.
  It would fail on the following input: Baz
         def thing(foo: Foo) = foo match {

This shows the developer what they have broken when they add a new product to the codebase. We’re using -Xfatal-warnings, otherwise this is just a warning.

However, the compiler will not perform exhaustivity checking if the class is not sealed or if there are guards, e.g.

  scala> def thing(foo: Foo) = foo match {
           case Bar(flag) if flag => true
  scala> thing(Baz)
  scala.MatchError: Baz (of class Baz$)
    at .thing(<console>:15)

To remain safe, don’t use guards on sealed types.

The -Xstrict-patmat-analysis flag has been proposed as a language improvement to perform additional pattern matcher checks.

4.1.4 Alternative Products and Coproducts

Another form of product is a tuple, which is like an unlabelled final case class.

(A.type, B, C) is equivalent to ABC in the above example but it is best to use final case class when part of an ADT because the lack of names is awkward to deal with.

Another form of coproduct is when we nest Either types. e.g.

  Either[X.type, Either[Y.type, Z]]

equivalent to the XYZ sealed abstract class. A cleaner syntax to define nested Either types is to create an alias type ending with a colon, allowing infix notation with association from the right:

  type |:[L,R] = Either[L, R]
  X.type |: Y.type |: Z

This is useful to create anonymous coproducts when you can’t put all the implementations into the same source file.

  type Accepted = String |: Long |: Boolean

Yet another alternative coproduct is to create a custom sealed abstract class with final case class definitions that simply wrap the desired type:

  sealed abstract class Accepted
  final case class AcceptedString(value: String) extends Accepted
  final case class AcceptedLong(value: Long) extends Accepted
  final case class AcceptedBoolean(value: Boolean) extends Accepted

Pattern matching on these forms of coproduct can be tedious, which is why Union Types are being explored in the Dotty next-generation scala compiler. Macros such as totalitarian and iotaz exist as alternative ways of encoding anonymous coproducts.

4.1.5 Convey Information

Besides being a container for necessary business information, data types can be used to encode constraints. For example,

  final case class NonEmptyList[A](head: A, tail: IList[A])

can never be empty. This makes scalaz.NonEmptyList a useful data type despite containing the same information as List.

Product types often contain types that are far more general than is allowed. In traditional OOP this would be handled with input validation through assertions:

  final case class Person(name: String, age: Int) {
    require(name.nonEmpty && age > 0) // breaks Totality, don't do this!

Instead, we can use the Either data type to provide Right[Person] for valid instances and protect invalid instances from propagating. Note that the constructor is private:

  final case class Person private(name: String, age: Int)
  object Person {
    def apply(name: String, age: Int): Either[String, Person] = {
      if (name.nonEmpty && age > 0) Right(new Person(name, age))
      else Left(s"bad input: $name, $age")
  def welcome(person: Person): String =
    s"${} you look wonderful at ${person.age}!"
  for {
    person <- Person("", -1)
  } yield welcome(person) Refined Data Types

A clean way to restrict the values of a general type is with the refined library, providing a suite of restrictions to the contents of data. To install refined, add the following to build.sbt

  libraryDependencies += "eu.timepit" %% "refined-scalaz" % "0.8.7"

and the following imports

  import eu.timepit.refined
  import refined.api.Refined

Refined allows us to define Person using adhoc refined types to capture requirements exactly (typically written A Refined B rather than Refined[A, B])

  import refined.numeric.Positive
  import refined.collection.NonEmpty
  final case class Person(
    name: String Refined NonEmpty,
    age: Int Refined Positive

A Refined B can be read as “an A that meets the requirements defined in B”. The underlying value can be obtained with .value. We can construct a value at runtime using .refineV

  scala> import refined.refineV
  scala> refineV[NonEmpty]("")
  Left(Predicate isEmpty() did not fail.)
  scala> refineV[NonEmpty]("Sam")

And if we add the following import


we can construct valid values at compiletime and get a compile error if the provided value does not meet the requirements

  scala> val sam: String Refined NonEmpty = "Sam"
  scala> val empty: String Refined NonEmpty = ""
  <console>:21: error: Predicate isEmpty() did not fail.

More complex requirements can be captured, for example we can use the built-in rule MaxSize with the following imports

  import refined.W
  import refined.boolean.And
  import refined.collection.MaxSize

capturing the requirement that the String must be both non-empty and have a maximum size of 10 characters:

  type Name = NonEmpty And MaxSize[W.`10`.T]
  final case class Person(
    name: String Refined Name,
    age: Int Refined Positive

It is easy to define custom requirements that are not covered by the refined library. For example, the requirement that a String contains a valid is as simple as

  final case class Url()
  object Url {
    implicit def urlValidate: refined.Validate.Plain[String, Url] =
      Validate.fromPartial(new, "Url", Url())

which can be used as String Refined Url.

4.1.6 Simple to Share

By not providing any functionality, ADTs can have a minimal set of dependencies. This makes them easy to publish and share with other developers. By using a simple data modelling language, it makes it possible to interact with cross-discipline teams, such as DBAs, UI developers and business analysts, using the actual code instead of a hand written document as the source of truth.

Furthermore, tooling can be more easily written to produce or consume schemas from other programming languages and wire protocols.

4.1.7 Counting Complexity

The complexity of a data type is the number of instances that can exist. A good data type has the least amount of complexity it needs to hold the information it conveys, and no more.

Values have a built-in complexity:

  • Unit has one instance (why it is called “unit”)
  • Boolean has two instances
  • Int has 4,294,967,295 instances
  • String has effectively infinite instances

To find the complexity of a product, we multiply the complexity of each part.

  • (Boolean, Boolean) has 4 instances (2*2)
  • (Boolean, Boolean, Boolean) has 8 instances (2*2*2)

To find the complexity of a coproduct, we add the complexity of each part.

  • (Boolean |: Boolean) has 4 instances (2+2)
  • (Boolean |: Boolean |: Boolean) has 6 instances (2+2+2)

To find the complexity of a GADT, multiply each part by the complexity of the type parameter:

  • Option[Boolean] has 3 instances, Some[Boolean] and None (2+1)

In FP, functions are total and must return an instance for every input, no Exception. Minimising the complexity of inputs and outputs is the best way to achieve totality. As a rule of thumb, it is a sign of a badly designed function when the complexity of a function’s return value is larger than the product of its inputs: it is a source of entropy.

The complexity of a total function itself is the number of possible functions that can satisfy the type signature: the output to the power of the input.

  • Unit => Boolean has complexity 2
  • Boolean => Boolean has complexity 4
  • Option[Boolean] => Option[Boolean] has complexity 27
  • Boolean => Int is a mere quintillion going on a sextillion.
  • Int => Boolean is so big that if all implementations were assigned a unique number, each would require 4 gigabytes to represent.

In reality, Int => Boolean will be something simple like isOdd, isEven or a sparse BitSet. This function, when used in an ADT, could be better replaced with a coproduct labelling the limited set of functions that are relevant.

When our complexity is “infinity in, infinity out” we should introduce restrictive data types and validation closer to the point of input with Refined from the previous section.

The ability to count the complexity of a type signature has one other practical application: we can find simpler type signatures with High School algebra! To go from a type signature to its algebra of complexity, simply replace

  • Either[A, B] with a + b
  • (A, B) with a * b
  • A => B with b ^ a

do some rearranging, and convert back. For example, say we’ve designed a framework based on callbacks and we’ve managed to work ourselves into the situation where we have created this type signature:

  (A => C) => ((B => C) => C)

We can convert and rearrange

  (c ^ (c ^ b)) ^ (c ^ a)
  = c ^ ((c ^ b) * (c ^ a))
  = c ^ (c ^ (a + b))

then convert back to types and get

  (Either[A, B] => C) => C

which is much simpler: we only need to ask the users of our framework to provide a Either[A, B] => C.

The same line of reasoning can be used to prove that

  A => B => C

is equivalent to

  (A, B) => C

also known as Currying.

4.1.8 Prefer Coproduct over Product

An archetypal modelling problem that comes up a lot is when there are mutually exclusive configuration parameters a, b and c. The product (a: Boolean, b: Boolean, c: Boolean) has complexity 8 whereas the coproduct

  sealed abstract class Config
  object Config {
    case object A extends Config
    case object B extends Config
    case object C extends Config

has a complexity of 3. It is better to model these configuration parameters as a coproduct rather than allowing 5 invalid states to exist.

The complexity of a data type also has implications on testing. It is practically impossible to test every possible input to a function, but it is easy to test a sample of values with the scalacheck property testing framework. If a random sample of a data type has a low probability of being valid, it is a sign that the data is modelled incorrectly.

4.1.9 Optimisations

A big advantage of using a simplified subset of the Scala language to represent data types is that tooling can optimise the JVM bytecode representation.

For example, we can pack Boolean and Option fields into an Array[Byte], cache instances, memoise hashCode, optimise equals, use @switch statements when pattern matching, and much more.

These optimisations are not applicable to OOP class hierarchies that may be managing state, throwing exceptions, or providing adhoc method implementations.

4.1.10 Generic Representation

We showed that product is synonymous with tuple and coproduct is synonymous with nested Either. The shapeless library takes this duality to the extreme and introduces a representation that is generic for all ADTs:

  • shapeless.HList (symbolically ::) for representing products (scala.Product already exists for another purpose)
  • shapeless.Coproduct (symbolically :+:) for representing coproducts

Shapeless provides the ability to convert back and forth between a generic representation and the ADT, allowing functions to be written that work for every final case class and sealed abstract class.

  scala> import shapeless._
         final case class Foo(a: String, b: Long)
         Generic[Foo].to(Foo("hello", 13L))
  res: String :: Long :: HNil = hello :: 13 :: HNil
  scala> Generic[Foo].from("hello" :: 13L :: HNil)
  res: Foo = Foo(hello,13)
  scala> sealed abstract class Bar
         case object Irish extends Bar
         case object English extends Bar
  scala> Generic[Bar].to(Irish)
  res: English.type :+: Irish.type :+: CNil = Inl(Irish)
  scala> Generic[Bar].from(Inl(Irish))
  res: Bar = Irish

HNil is the empty product and CNil is the empty coproduct.

It is not necessary to know how to write generic code to be able to make use of shapeless. However, it is an important part of FP Scala so we will return to it later with a dedicated chapter.

4.2 Functionality

Pure functions are typically defined as methods on an object.

  package object math {
    def sin(x: Double): Double = java.lang.Math.sin(x)

However, it can be clunky to use object methods since it reads inside-out, not left to right. In addition, a function on an object steals the namespace. If we were to define sin(t: T) somewhere else we get ambiguous reference errors. This is the same problem as Java’s static methods vs class methods.

With the implicit class language feature (also known as extension methodology or syntax), and a little boilerplate, we can get the familiar style:

  scala> implicit class DoubleOps(x: Double) {
           def sin: Double = math.sin(x)
  scala> 1.0.sin
  res: Double = 0.8414709848078965

Often it is best to just skip the object definition and go straight for an implicit class, keeping boilerplate to a minimum:

  implicit class DoubleOps(x: Double) {
    def sin: Double = java.lang.Math.sin(x)

4.2.1 Polymorphic Functions

The more common kind of function is a polymorphic function, which lives in a typeclass. A typeclass is a trait that:

  • holds no state
  • has a type parameter
  • has at least one abstract method
  • may contain generalised methods
  • may extend other typeclasses

Typeclasses are used in the Scala stdlib. We’ll explore a simplified version of scala.math.Numeric to demonstrate the principle:

  trait Ordering[T] {
    def compare(x: T, y: T): Int
    def lt(x: T, y: T): Boolean = compare(x, y) < 0
    def gt(x: T, y: T): Boolean = compare(x, y) > 0
  trait Numeric[T] extends Ordering[T] {
    def plus(x: T, y: T): T
    def times(x: T, y: T): T
    def negate(x: T): T
    def zero: T
    def abs(x: T): T = if (lt(x, zero)) negate(x) else x

We can see all the key features of a typeclass in action:

  • there is no state
  • Ordering and Numeric have type parameter T
  • Ordering has abstract compare and Numeric has abstract plus, times, negate and zero
  • Ordering defines generalised lt and gt based on compare, Numeric defines abs in terms of lt, negate and zero.
  • Numeric extends Ordering

We can now write functions for types that “have a” Numeric typeclass:

  def signOfTheTimes[T](t: T)(implicit N: Numeric[T]): T = {
    import N._
    times(negate(abs(t)), t)

We are no longer dependent on the OOP hierarchy of our input types, i.e. we don’t demand that our input “is a” Numeric, which is vitally important if we want to support a third party class that we cannot redefine.

Another advantage of typeclasses is that the association of functionality to data is at compiletime, as opposed to OOP runtime dynamic dispatch.

For example, whereas the List class can only have one implementation of a method, a typeclass method allows us to have a different implementation depending on the List contents and therefore offload work to compiletime instead of leaving it to runtime.

4.2.2 Syntax

The syntax for writing signOfTheTimes is clunky, there are some things we can do to clean it up.

Downstream users will prefer to see our method use context bounds, since the signature reads cleanly as “takes a T that has a Numeric

  def signOfTheTimes[T: Numeric](t: T): T = ...

but now we have to use implicitly[Numeric[T]] everywhere. By defining boilerplate on the companion of the typeclass

  object Numeric {
    def apply[T](implicit numeric: Numeric[T]): Numeric[T] = numeric

we can obtain the implicit with less noise

  def signOfTheTimes[T: Numeric](t: T): T = {
    val N = Numeric[T]
    import N._
    times(negate(abs(t)), t)

But it is still worse for us as the implementors. We have the syntactic problem of inside-out static methods vs class methods. We deal with this by introducing ops on the typeclass companion:

  object Numeric {
    def apply[T](implicit numeric: Numeric[T]): Numeric[T] = numeric
    object ops {
      implicit class NumericOps[T](t: T)(implicit N: Numeric[T]) {
        def +(o: T): T =, o)
        def *(o: T): T = N.times(t, o)
        def unary_-: T = N.negate(t)
        def abs: T = N.abs(t)
        // duplicated from Ordering.ops
        def <(o: T): T =, o)
        def >(o: T): T =, o)

Note that -x is expanded into x.unary_- by the compiler’s syntax sugar, which is why we define unary_- as an extension method. We can now write the much cleaner:

  import Numeric.ops._
  def signOfTheTimes[T: Numeric](t: T): T = -(t.abs) * t

The good news is that we never need to write this boilerplate because Simulacrum provides a @typeclass macro annotation to have the companion apply and ops automatically generated. It even allows us to define alternative (usually symbolic) names for common methods. In full:

  import simulacrum._
  @typeclass trait Ordering[T] {
    def compare(x: T, y: T): Int
    @op("<") def lt(x: T, y: T): Boolean = compare(x, y) < 0
    @op(">") def gt(x: T, y: T): Boolean = compare(x, y) > 0
  @typeclass trait Numeric[T] extends Ordering[T] {
    @op("+") def plus(x: T, y: T): T
    @op("*") def times(x: T, y: T): T
    @op("unary_-") def negate(x: T): T
    def zero: T
    def abs(x: T): T = if (lt(x, zero)) negate(x) else x
  import Numeric.ops._
  def signOfTheTimes[T: Numeric](t: T): T = -(t.abs) * t

4.2.3 Instances

Instances of Numeric (which are also instances of Ordering) are defined as an implicit val that extends the typeclass, and can provide optimised implementations for the generalised methods:

  implicit val NumericDouble: Numeric[Double] = new Numeric[Double] {
    def plus(x: Double, y: Double): Double = x + y
    def times(x: Double, y: Double): Double = x * y
    def negate(x: Double): Double = -x
    def zero: Double = 0.0
    def compare(x: Double, y: Double): Int =, y)
    // optimised
    override def lt(x: Double, y: Double): Boolean = x < y
    override def gt(x: Double, y: Double): Boolean = x > y
    override def abs(x: Double): Double = java.lang.Math.abs(x)

Although we are using +, *, unary_-, < and > here, which are the ops (and could be an infinite loop!), these methods exist already on Double. Class methods are always used in preference to extension methods. Indeed, the scala compiler performs special handling of primitives and converts these method calls into raw dadd, dmul, dcmpl and dcmpg bytecode instructions, respectively.

We can also implement Numeric for Java’s BigDecimal class (avoid scala.BigDecimal, it is fundamentally broken)

  import java.math.{ BigDecimal => BD }
  implicit val NumericBD: Numeric[BD] = new Numeric[BD] {
    def plus(x: BD, y: BD): BD = x.add(y)
    def times(x: BD, y: BD): BD = x.multiply(y)
    def negate(x: BD): BD = x.negate
    def zero: BD = BD.ZERO
    def compare(x: BD, y: BD): Int = x.compareTo(y)

We could even take some liberties and create our own data structure for complex numbers:

  final case class Complex[T](r: T, i: T)

And derive a Numeric[Complex[T]] if Numeric[T] exists. Since these instances depend on the type parameter, it is a def, not a val.

  implicit def numericComplex[T: Numeric]: Numeric[Complex[T]] =
    new Numeric[Complex[T]] {
      type CT = Complex[T]
      def plus(x: CT, y: CT): CT = Complex(x.r + y.r, x.i + y.i)
      def times(x: CT, y: CT): CT =
        Complex(x.r * y.r + (-x.i * y.i), x.r * y.i + x.i * y.r)
      def negate(x: CT): CT = Complex(-x.r, -x.i)
      def zero: CT = Complex(Numeric[T].zero, Numeric[T].zero)
      def compare(x: CT, y: CT): Int = {
        val real = (Numeric[T].compare(x.r, y.r))
        if (real != 0) real
        else Numeric[T].compare(x.i, y.i)

The observant reader may notice that abs is not at all what a mathematician would expect. The correct return value for abs should be T, not Complex[T].

scala.math.Numeric tries to do too much and does not generalise beyond real numbers. This is a good lesson that smaller, well defined, typeclasses are often better than a monolithic collection of overly specific features.

If you need to write generic code that works for a wide range of number types, prefer spire to the stdlib. Indeed, in the next chapter we will see that concepts such as having a zero element, or adding two values, are worthy of their own typeclass.

4.2.4 Implicit Resolution

We’ve discussed implicits a lot: this section is to clarify what implicits are and how they work.

Implicit parameters are when a method requests that a unique instance of a particular type is in the implicit scope of the caller, with special syntax for typeclass instances. Implicit parameters are a clean way to thread configuration through an application.

In this example, foo requires that typeclasses for Numeric and shapeless’ Typeable are available for T, as well as an implicit (user-defined) Config object.

  def foo[T: Numeric: Typeable](implicit conf: Config) = ...

Implicit conversion is when an implicit def exists. One such use of implicit conversions is to enable extension methodology. When the compiler is resolving a call to a method, it first checks if the method exists on the type, then its ancestors (Java-like rules). If it fails to find a match, it will search the implicit scope for conversions to other types, then search for methods on those types.

Another use for implicit conversion is typeclass derivation. In the previous section we wrote an implicit def that derived a Numeric[Complex[T]] if a Numeric[T] is in the implicit scope. It is possible to chain together many implicit def (including recursively) which is the basis of typeful programming, allowing for computations to be performed at compiletime rather than runtime.

The glue that combines implicit parameters (receivers) with implicit conversion (providers) is implicit resolution.

First, the normal variable scope is searched for implicits, in order:

  • local scope, including scoped imports (e.g. the block or method)
  • outer scope, including scoped imports (e.g. members in the class)
  • ancestors (e.g. members in the super class)
  • the current package object
  • ancestor package objects (only when using nested packages)
  • the file’s imports

If that fails to find a match, the special scope is searched, which looks for implicit instances inside a type’s companion, its package object, outer objects (if nested), and then repeated for ancestors. This is performed, in order, for the:

  • given parameter type
  • expected parameter type
  • type parameter (if there is one)

If two matching implicits are found in the same phase of implicit resolution, an ambiguous implicit error is raised.

Implicits are often defined on a trait, which is then extended by an object. This is to try and control the priority of an implicit relative to another more specific one, to avoid ambiguous implicits.

The Scala Language Specification is rather vague for corner cases, and the compiler implementation is the de facto standard. There are some rules of thumb that we will use throughout this book, e.g. prefer implicit val over implicit object despite the temptation of less typing. It is a quirk of implicit resolution that implicit object on companion objects are not treated the same as implicit val.

Implicit resolution falls short when there is a hierarchy of typeclasses, like Ordering and Numeric. If we write a function that takes an implicit Ordering, and we call it for a type which has an instance of Numeric defined on the Numeric companion, the compiler will fail to find it. A workaround is to add implicit conversions to the companion of Ordering that up-cast more specific instances. Fixed In Dotty.

Implicit resolution is particularly hit-or-miss if type aliases are used where the shape of the implicit parameters are changed. For example an implicit parameter using an alias such as type Values[A] = List[Option[A]] will probably fail to find implicits defined as raw List[Option[A]] because the shape is changed from a thing of things of A to a thing of A.

4.3 Modelling OAuth2

We will finish this chapter with a practical example of data modelling and typeclass derivation, combined with algebra / module design from the previous chapter.

In our drone-dynamic-agents application, we must communicate with Drone and Google Cloud using JSON over REST. Both services use OAuth2 for authentication. Although there are many ways to interpret OAuth2, we’ll focus on the version that works for Google Cloud (the Drone version is even simpler).

4.3.1 Description

Every Google Cloud application needs to have an OAuth 2.0 Client Key set up at{PROJECT_ID}

You will be provided with a Client ID and a Client secret.

The application can then obtain a one time code by making the user perform an Authorization Request in their browser (yes, really, in their browser). We need to make this page open in the browser:\

The code is delivered to the {CALLBACK_URI} in a GET request. To capture it in our application, we need to have a web server listening on localhost.

Once we have the code, we can perform an Access Token Request:

  POST /oauth2/v4/token HTTP/1.1
  Content-length: {CONTENT_LENGTH}
  content-type: application/x-www-form-urlencoded
  user-agent: google-oauth-playground

which gives a JSON response payload

    "access_token": "BEARER_TOKEN",
    "token_type": "Bearer",
    "expires_in": 3600,
    "refresh_token": "REFRESH_TOKEN"

Bearer tokens typically expire after an hour, and can be refreshed by sending an HTTP request with any valid refresh token:

  POST /oauth2/v4/token HTTP/1.1
  Content-length: {CONTENT_LENGTH}
  content-type: application/x-www-form-urlencoded
  user-agent: google-oauth-playground

responding with

    "access_token": "BEARER_TOKEN",
    "token_type": "Bearer",
    "expires_in": 3600

Google expires all but the most recent 50 bearer tokens, so the expiry times are just guidance. The refresh tokens persist between sessions and can be expired manually by the user. We can therefore have a one-time setup application to obtain the refresh token and then include the refresh token as configuration for the user’s install of the headless server.

4.3.2 Data

The first step is to model the data needed for OAuth2. We create an ADT with fields having exactly the same name as required by the OAuth2 server. We will use String and Long for brevity, but we could use refined types if they leak into our business models.

  package http.oauth2.client.api
  import refined.api.Refined
  import refined.string.Url
  final case class AuthRequest(
    redirect_uri: String Refined Url,
    scope: String,
    client_id: String,
    prompt: String = "consent",
    response_type: String = "code",
    access_type: String = "offline"
  final case class AccessRequest(
    code: String,
    redirect_uri: String Refined Url,
    client_id: String,
    client_secret: String,
    scope: String = "",
    grant_type: String = "authorization_code"
  final case class AccessResponse(
    access_token: String,
    token_type: String,
    expires_in: Long,
    refresh_token: String
  final case class RefreshRequest(
    client_secret: String,
    refresh_token: String,
    client_id: String,
    grant_type: String = "refresh_token"
  final case class RefreshResponse(
    access_token: String,
    token_type: String,
    expires_in: Long

4.3.3 Functionality

We need to marshal the data classes we defined in the previous section into JSON, URLs and POST-encoded forms. Since this requires polymorphism, we will need typeclasses.

spray-json gives us an ADT for JSON and typeclasses to convert to/from that ADT (paraphrased for brevity):

  package spray.json
  sealed abstract class JsValue
  case object JsNull extends JsValue
  final case class JsBoolean(value: Boolean) extends JsValue
  final case class JsNumber(value: BigDecimal) extends JsValue
  final case class JsString(value: String) extends JsValue
  final case class JsArray(value: Vector[JsValue]) extends JsValue
  final case class JsObject(fields: Map[String, JsValue]) extends JsValue
  @typeclass trait JsonWriter[T] {
    def toJson(t: T): JsValue
  @typeclass trait JsonReader[T] {
    def fromJson(j: JsValue): T

To depend on spray-json in our project we must add the following to build.sbt:

  libraryDependencies += "xyz.driver" %% "spray-json-derivation" % "0.4.1"

Because spray-json-derivation provides derived typeclass instances, we can conjure up a JsonReader[AccessResponse] and JsonReader[RefreshResponse]. This is an example of parsing text into AccessResponse:

  scala> import spray.json.ImplicitDerivedFormats._
         for {
           json     <- spray.json.JsonParser("""
                         "access_token": "BEARER_TOKEN",
                         "token_type": "Bearer",
                         "expires_in": 3600,
                         "refresh_token": "REFRESH_TOKEN"
           response <- JsonReader[AccessResponse].fromJson(json)
         } yield response
  res = AccessResponse(BEARER_TOKEN,Bearer,3600,REFRESH_TOKEN)

We need to write our own typeclasses for URL and POST encoding. The following is a reasonable design:

  package http.encoding
  final case class UrlQuery(params: List[(String, String)]) {
    def forUrl(url: String Refined Url): String Refined Url = ...
  @typeclass trait UrlQueryWriter[A] {
    def toUrlQuery(a: A): UrlQuery
  @typeclass trait UrlEncodedWriter[A] {
    def toUrlEncoded(a: A): String

We need to provide typeclass instances for basic types:

  object UrlEncodedWriter {
    import ops._
    implicit val string: UrlEncodedWriter[String] =
      { s =>, "UTF-8") }
    implicit val long: UrlEncodedWriter[Long] = _.toString
    implicit val stringySeq: UrlEncodedWriter[Seq[(String, String)]] = { case (k, v) => s"${k.toUrlEncoded}=${v.toUrlEncoded}" }.mkString("&")
    implicit val url: UrlEncodedWriter[String Refined Url] =
      { s =>, "UTF-8") }

In a dedicated chapter on Typeclass Derivation we will calculate instances of UrlQueryWriter and UrlEncodedWriter automatically, but for now we will write the boilerplate for the types we wish to convert:

  import UrlEncodedWriter.ops._
  object AuthRequest {
    private def stringify[T: UrlEncodedWriter](t: T) =, "UTF-8")
    implicit val query: UrlQueryWriter[AuthRequest] = { a =>
        ("redirect_uri"  -> stringify(a.redirect_uri)),
        ("scope"         -> stringify(a.scope)),
        ("client_id"     -> stringify(a.client_id)),
        ("prompt"        -> stringify(a.prompt)),
        ("response_type" -> stringify(a.response_type)),
        ("access_type"   -> stringify(a.access_type)))
  object AccessRequest {
    implicit val encoded: UrlEncodedWriter[AccessRequest] = { a =>
        "code"          -> a.code.toUrlEncoded,
        "redirect_uri"  -> a.redirect_uri.toUrlEncoded,
        "client_id"     -> a.client_id.toUrlEncoded,
        "client_secret" -> a.client_secret.toUrlEncoded,
        "scope"         -> a.scope.toUrlEncoded,
        "grant_type"    -> a.grant_type.toUrlEncoded
  object RefreshRequest {
    implicit val encoded: UrlEncodedWriter[RefreshRequest] = { r =>
        "client_secret" -> r.client_secret.toUrlEncoded,
        "refresh_token" -> r.refresh_token.toUrlEncoded,
        "client_id"     -> r.client_id.toUrlEncoded,
        "grant_type"    -> r.grant_type.toUrlEncoded

4.3.4 Module

That concludes the data and functionality modelling required to implement OAuth2. Recall from the previous chapter that we define mockable components that need to interact with the world as algebras, and we define pure business logic in a module.

We define our dependency algebras, and use context bounds to show that our responses must have a JsonReader and our POST payload must have a UrlEncodedWriter:

  package http.client.algebra
  final case class Response[T](header: HttpResponseHeader, body: T)
  trait JsonHttpClient[F[_]] {
    def get[B: JsonReader](
      uri: String Refined Url,
      headers: List[HttpHeader] = Nil
    ): F[Response[B]]
    def postUrlencoded[A: UrlEncoded, B: JsonReader](
      uri: String Refined Url,
      payload: A,
      headers: List[HttpHeader] = Nil
    ): F[Response[B]]
  package http.oauth2.client.algebra
  final case class CodeToken(token: String, redirect_uri: String Refined Url)
  trait UserInteraction[F[_]] {
    /** returns the URL of the local server */
    def start: F[String Refined Url]
    /** prompts the user to open this URL */
    def open(uri: String Refined Url): F[Unit]
    /** recover the code from the callback */
    def stop: F[CodeToken]
  trait LocalClock[F[_]] {
    def now: F[java.time.LocalDateTime]

some convenient data classes

  final case class ServerConfig(
    auth: String Refined Url,
    access: String Refined Url,
    refresh: String Refined Url,
    scope: String,
    clientId: String,
    clientSecret: String
  final case class RefreshToken(token: String)
  final case class BearerToken(token: String, expires: LocalDateTime)

and then write an OAuth2 client:

  import java.time.temporal.ChronoUnit
  import http.encoding.UrlQueryWriter.ops._
  import spray.json.ImplicitDerivedFormats._
  class OAuth2Client[F[_]: Monad](
    config: ServerConfig
    user: UserInteraction[F],
    client: JsonHttpClient[F],
    clock: LocalClock[F]
  ) {
    def authenticate: F[CodeToken] =
      for {
        callback <- user.start
        params   = AuthRequest(callback, config.scope, config.clientId)
        _        <-
        code     <- user.stop
      } yield code
    def access(code: CodeToken): F[(RefreshToken, BearerToken)] =
      for {
        request <- AccessRequest(code.token,
        response <- client.postUrlencoded[AccessRequest, AccessResponse](
                     config.access, request)
        time    <-
        msg     = response.body
        expires =, ChronoUnit.SECONDS)
        refresh = RefreshToken(msg.refresh_token)
        bearer  = BearerToken(msg.access_token, expires)
      } yield (refresh, bearer)
    def bearer(refresh: RefreshToken): F[BearerToken] =
      for {
        request <- RefreshRequest(config.clientSecret,
        response <- client.postUrlencoded[RefreshRequest, RefreshResponse](
                     config.refresh, request)
        time    <-
        msg     = response.body
        expires =, ChronoUnit.SECONDS)
        bearer  = BearerToken(msg.access_token, expires)
      } yield bearer

4.4 Summary

  • data types are defined as products (final case class) and coproducts (sealed abstract class).
  • Refined types can enforce constraints on values
  • specific functions are defined on object or implicit class, according to personal taste.
  • polymorphic functions are defined as typeclasses. Functionality is provided via “has a” context bounds, rather than “is a” class hierarchies.
  • typeclass instances are implementations of the typeclass.
  • @simulacrum.typeclass generates .ops on the companion, providing convenient syntax for types that have a typeclass instance.
  • typeclass derivation is compiletime composition of typeclass instances.
  • generic instances automatically derive instances for your data types.

5. Scalaz Typeclasses

In this chapter we will tour most of the typeclasses in scalaz-core. We don’t use everything in drone-dynamic-agents so we will give standalone examples when appropriate.

There has been criticism of the naming in scalaz, and functional programming in general. Most names follow the conventions introduced in the Haskell programming language, based on Category Theory. Feel free to set up type aliases in your own codebase if you would prefer to use verbs based on the primary functionality of the typeclass (e.g. Mappable, Pureable, FlatMappable) until you are comfortable with the standard names.

Before we introduce the typeclass hierarchy, we will peek at the four most important methods from a control flow perspective: the methods we will use the most in typical FP applications:

Typeclass Method From Given To
Functor map F[A] A => B F[B]
Applicative pure A   F[A]
Monad flatMap F[A] A => F[B] F[B]
Traverse traverse F[A] A => G[B] G[F[B]]

We know that operations which return a F[_] can be run sequentially in a for comprehension by .flatMap, defined on its Monad[F]. The context F[_] can be thought of as a container for an intentional effect with A as the output: flatMap allows us to generate new effects F[B] at runtime based on the results of evaluating previous effects.

Of course, not all type constructors F[_] are effectful, even if they have a Monad[F]. Often they are data structures. By using the least specific abstraction, we can reuse code for List, Either, Future and more.

If we only need to transform the output from an F[_], that’s just map, introduced by Functor. In Chapter 3, we ran effects in parallel by creating a product and mapping over them. In Functional Programming, parallelisable computations are considered less powerful than sequential ones.

In between Monad and Functor is Applicative, defining pure that lets us lift a value into an effect, or create a data structure from a single value.

traverse is useful for rearranging type constructors. If you find yourself with an F[G[_]] but you really need a G[F[_]] then you need Traverse. For example, say you have a List[Future[Int]] but you need it to be a Future[List[Int]], just call .traverse(identity), or its simpler sibling .sequence.

5.1 Agenda

This chapter is longer than usual and jam-packed with information: it is perfectly reasonable to attack it over several sittings. You are not expected to remember everything (doing so would require super-human powers) so treat this chapter as a way of knowing where to look for more information.

Notably absent are typeclasses that extend Monad, which get their own chapter later.

Scalaz uses code generation, not simulacrum. However, for brevity, we present code snippets with @typeclass. Equivalent syntax is available when we import scalaz._, Scalaz._

5.2 Appendable Things

  @typeclass trait Semigroup[A] {
    @op("|+|") def append(x: A, y: =>A): A
    def multiply1(value: F, n: Int): F = ...
  @typeclass trait Monoid[A] extends Semigroup[A] {
    def zero: A
    def multiply(value: F, n: Int): F =
      if (n <= 0) zero else multiply1(value, n - 1)
  @typeclass trait Band[A] extends Semigroup[A]

A Semigroup should exist for a type if two elements can be combined to produce another element of the same type. The operation must be associative, meaning that the order of nested operations should not matter, i.e.

  (a |+| b) |+| c == a |+| (b |+| c)
  (1 |+| 2) |+| 3 == 1 |+| (2 |+| 3)

A Monoid is a Semigroup with a zero element (also called empty or identity). Combining zero with any other a should give a.

  a |+| zero == a
  a |+| 0 == a

This is probably bringing back memories of Numeric from Chapter 4, which tried to do too much and was unusable beyond the most basic of number types. There are implementations of Monoid for all the primitive numbers, but the concept of appendable things is useful beyond numbers.

  scala> "hello" |+| " " |+| "world!"
  res: String = "hello world!"
  scala> List(1, 2) |+| List(3, 4)
  res: List[Int] = List(1, 2, 3, 4)

Band has the law that the append operation of the same two elements is idempotent, i.e. gives the same value. Examples are anything that can only be one value, such as Unit, least upper bounds, or a Set. Band provides no further methods yet users can make use of the guarantees for performance optimisation.

As a realistic example for Monoid, consider a trading system that has a large database of reusable trade templates. Creating the default values for a new trade involves selecting and combining templates with a “last rule wins” merge policy (e.g. if templates have a value for the same field).

We’ll create a simple template schema to demonstrate the principle, but keep in mind that a realistic system would have a more complicated ADT.

  sealed abstract class Currency
  case object EUR extends Currency
  case object USD extends Currency
  final case class TradeTemplate(
    payments: List[java.time.LocalDate],
    ccy: Option[Currency],
    otc: Option[Boolean]

If we write a method that takes templates: List[TradeTemplate], we only need to call

  val zero = Monoid[TradeTemplate].zero
  templates.foldLeft(zero)(_ |+| _)

and our job is done!

But to get zero or call |+| we must have an instance of Monoid[TradeTemplate]. Although we will generically derive this in a later chapter, for now we’ll create an instance on the companion:

  object TradeTemplate {
    implicit val monoid: Monoid[TradeTemplate] = Monoid.instance(
      (a, b) => TradeTemplate(a.payments |+| b.payments,
                              a.ccy |+| b.ccy,
                              a.otc |+| b.otc),
      TradeTemplate(Nil, None, None)

However, this doesn’t do what we want because Monoid[Option[A]] will append its contents, e.g.

  scala> Option(2) |+| None
  res: Option[Int] = Some(2)
  scala> Option(2) |+| Option(1)
  res: Option[Int] = Some(3)

whereas we want “last rule wins”. We can override the default Monoid[Option[A]] with our own:

  implicit def lastWins[A]: Monoid[Option[A]] = Monoid.instance(
      case (None, None)   => None
      case (only, None)   => only
      case (None, only)   => only
      case (_   , winner) => winner

Now everything compiles, let’s try it out…

  scala> import java.time.{LocalDate => LD}
  scala> val templates = List(
           TradeTemplate(Nil,                     None,      None),
           TradeTemplate(Nil,                     Some(EUR), None),
           TradeTemplate(List(LD.of(2017, 8, 5)), Some(USD), None),
           TradeTemplate(List(LD.of(2017, 9, 5)), None,      Some(true)),
           TradeTemplate(Nil,                     None,      Some(false))
  scala> templates.foldLeft(zero)(_ |+| _)
  res: TradeTemplate = TradeTemplate(

All we needed to do was implement one piece of business logic and Monoid took care of everything else for us!

Note that the list of payments are concatenated. This is because the default Monoid[List] uses concatenation of elements and happens to be the desired behaviour. If the business requirement was different, it would be a simple case of providing a custom Monoid[List[LocalDate]]. Recall from Chapter 4 that with compiletime polymorphism we can have a different implementation of append depending on the E in List[E], not just the base runtime class List.

5.3 Objecty Things

In the chapter on Data and Functionality we said that the JVM’s notion of equality breaks down for many things that we can put into an ADT. The problem is that the JVM was designed for Java, and equals is defined on java.lang.Object whether it makes sense or not. There is no way to remove equals and no way to guarantee that it is implemented.

However, in FP we prefer typeclasses for polymorphic functionality and even concepts as simple equality are captured at compiletime.

  @typeclass trait Equal[F]  {
    @op("===") def equal(a1: F, a2: F): Boolean
    @op("/==") def notEqual(a1: F, a2: F): Boolean = !equal(a1, a2)

Indeed === (triple equals) is more typesafe than == (double equals) because it can only be compiled when the types are the same on both sides of the comparison. You’d be surprised how many bugs this catches.

equal has the same implementation requirements as Object.equals

  • commutative f1 === f2 implies f2 === f1
  • reflexive f === f
  • transitive f1 === f2 && f2 === f3 implies f1 === f3

By throwing away the universal concept of Object.equals we don’t take equality for granted when we construct an ADT, stopping us at compiletime from expecting equality when there is none.

Continuing the trend of replacing old Java concepts, rather than data being a java.lang.Comparable, they now have an Order according to:

  @typeclass trait Order[F] extends Equal[F] {
    @op("?|?") def order(x: F, y: F): Ordering
    override  def equal(x: F, y: F): Boolean = ...
    @op("<" ) def lt(x: F, y: F): Boolean = ...
    @op("<=") def lte(x: F, y: F): Boolean = ...
    @op(">" ) def gt(x: F, y: F): Boolean = ...
    @op(">=") def gte(x: F, y: F): Boolean = ...
    def max(x: F, y: F): F = ...
    def min(x: F, y: F): F = ...
    def sort(x: F, y: F): (F, F) = ...
  sealed abstract class Ordering
  object Ordering {
    case object LT extends Ordering
    case object EQ extends Ordering
    case object GT extends Ordering

Things that have an order may also be discrete, allowing us to walk successors and predecessors:

  @typeclass trait Enum[F] extends Order[F] {
    def succ(a: F): F
    def pred(a: F): F
    def min: Option[F]
    def max: Option[F]
    @op("-+-") def succn(n: Int, a: F): F = ...
    @op("---") def predn(n: Int, a: F): F = ...
    @op("|->" ) def fromToL(from: F, to: F): List[F] = ...
    @op("|-->") def fromStepToL(from: F, step: Int, to: F): List[F] = ...
    @op("|=>" ) def fromToL(from: F, to: F): EphemeralStream[F] = ...
    @op("|==>") def fromStepToL(from: F, step: Int, to: F): EphemeralStream[F] = ...
  scala> 10 |--> (2, 20)
  res: List[Int] = List(10, 12, 14, 16, 18, 20)
  scala> 'm' |-> 'u'
  res: List[Char] = List(m, n, o, p, q, r, s, t, u)

We’ll discuss EphemeralStream in the next chapter, for now you just need to know that it is a potentially infinite data structure that avoids memory retention problems in the stdlib Stream.

Similarly to Object.equals, the concept of a .toString on every class does not make sense in Java. We would like to enforce stringyness at compiletime and this is exactly what Show achieves:

  trait Show[F] {
    def show(f: F): Cord = ...
    def shows(f: F): String = ...

We’ll explore Cord in more detail in the chapter on data types, you need only know that it is an efficient data structure for storing and manipulating String.

Unfortunately, due to Scala’s default implicit conversions in Predef, and language level support for toString in interpolated strings, it can be incredibly hard to remember to use shows instead of toString.

5.4 Mappable Things

We’re focusing on things that can be mapped over, or traversed, in some sense:

5.4.1 Functor

  @typeclass trait Functor[F[_]] {
    def map[A, B](fa: F[A])(f: A => B): F[B]
    def void[A](fa: F[A]): F[Unit] = map(fa)(_ => ())
    def fproduct[A, B](fa: F[A])(f: A => B): F[(A, B)] = map(fa)(a => (a, f(a)))
    def fpair[A](fa: F[A]): F[(A, A)] = map(fa)(a => (a, a))
    def strengthL[A, B](a: A, f: F[B]): F[(A, B)] = map(f)(b => (a, b))
    def strengthR[A, B](f: F[A], b: B): F[(A, B)] = map(f)(a => (a, b))
    def lift[A, B](f: A => B): F[A] => F[B] = map(_)(f)
    def mapply[A, B](a: A)(f: F[A => B]): F[B] = map(f)((ff: A => B) => ff(a))

The only abstract method is map, and it must compose, i.e. mapping with f and then again with g is the same as mapping once with the composition of f and g: ==

The map should also perform a no-op if the provided function is identity (i.e. x => x) == fa => x) == fa

Functor defines some convenience methods around map that can be optimised by specific instances. The documentation has been intentionally omitted in the above definitions to encourage you to guess what a method does before looking at the implementation. Please spend a moment studying only the type signature of the following before reading further:

  def void[A](fa: F[A]): F[Unit]
  def fproduct[A, B](fa: F[A])(f: A => B): F[(A, B)]
  def fpair[A](fa: F[A]): F[(A, A)]
  def strengthL[A, B](a: A, f: F[B]): F[(A, B)]
  def strengthR[A, B](f: F[A], b: B): F[(A, B)]
  // harder
  def lift[A, B](f: A => B): F[A] => F[B]
  def mapply[A, B](a: A)(f: F[A => B]): F[B]
  1. void takes an instance of the F[A] and always returns an F[Unit], it forgets all the values whilst preserving the structure.
  2. fproduct takes the same input as map but returns F[(A, B)], i.e. it tuples the contents with the result of applying the function. This is useful when we wish to retain the input.
  3. fpair twins all the elements of A into a tuple F[(A, A)]
  4. strengthL pairs the contents of an F[B] with a constant A on the left.
  5. strengthR pairs the contents of an F[A] with a constant B on the right.
  6. lift takes a function A => B and returns a F[A] => F[B]. In other words, it takes a function over the contents of an F[A] and returns a function that operates on the F[A] directly.
  7. mapply is a mind bender. Say you have an F[_] of functions A => B and a value A, then you can get an F[B]. It has a similar signature to pure but requires the caller to provide the F[A => B].

fpair, strengthL and strengthR look pretty useless, but they are useful when we wish to retain some information that would otherwise be lost to scope.

Functor has some special syntax:

  implicit class FunctorOps[F[_]: Functor, A](self: F[A]) {
    def as[B](b: =>B): F[B] = Functor[F].map(self)(_ => b)
    def >|[B](b: =>B): F[B] = as(b)

as and >| are a way of replacing the output with a constant.

In our example application, as a nasty hack (which we didn’t even admit to until now), we defined start and stop to return their input:

  def start(node: MachineNode): F[MachineNode]
  def stop (node: MachineNode): F[MachineNode]

This allowed us to write terse business logic such as

  for {
    _      <- m.start(node)
    update = world.copy(pending = Map(node -> world.time))
  } yield update


  for {
    stopped <- nodes.traverse(m.stop)
    updates = -> world.time).toList.toMap
    update  = world.copy(pending = world.pending ++ updates)
  } yield update

But this hack pushes unnecessary complexity into the implementations. It is better if we let our algebras return F[Unit] and use as:

  m.start(node) as world.copy(pending = Map(node -> world.time))


  for {
    stopped <- nodes.traverse(a => m.stop(a) as a)
    updates = -> world.time).toList.toMap
    update  = world.copy(pending = world.pending ++ updates)
  } yield update

As a bonus, we are now using the less powerful Functor instead of Monad when starting a node.

5.4.2 Foldable

Technically, Foldable is for data structures that can be walked to produce a summary value. However, this undersells the fact that it is a one-typeclass army that can provide most of what you’d expect to see in a Collections API.

There are so many methods we are going to have to split them out, beginning with the abstract methods:

  @typeclass trait Foldable[F[_]] {
    def foldMap[A, B: Monoid](fa: F[A])(f: A => B): B
    def foldRight[A, B](fa: F[A], z: =>B)(f: (A, =>B) => B): B
    def foldLeft[A, B](fa: F[A], z: B)(f: (B, A) => B): B = ...

An instance of Foldable need only implement foldMap and foldRight to get all of the functionality in this typeclass, although methods are typically optimised for specific data structures.

You might recognise foldMap by its marketing buzzword name, MapReduce. Given an F[A], a function from A to B, and a way to combine B (provided by the Monoid, along with a zero B), we can produce a summary value of type B. There is no enforced operation order, allowing for parallel computation.

foldRight does not require its parameters to have a Monoid, meaning that it needs a starting value z and a way to combine each element of the data structure with the summary value. The order for traversing the elements is from right to left and therefore it cannot be parallelised.

foldLeft traverses elements from left to right. foldLeft can be implemented in terms of foldMap, but most instances choose to implement it because it is such a basic operation. Since it is usually implemented with tail recursion, there are no byname parameters.

The only law for Foldable is that foldLeft and foldRight should each be consistent with foldMap for monoidal operations. e.g. appending an element to a list for foldLeft and prepending an element to a list for foldRight. However, foldLeft and foldRight do not need to be consistent with each other: in fact they often produce the reverse of each other.

The simplest thing to do with foldMap is to use the identity function, giving fold (the natural sum of the monoidal elements), with left/right variants to allow choosing based on performance criteria:

  def fold[A: Monoid](t: F[A]): A = ...
  def sumr[A: Monoid](fa: F[A]): A = ...
  def suml[A: Monoid](fa: F[A]): A = ...

Recall that when we learnt about Monoid, we wrote this:

  scala> templates.foldLeft(Monoid[TradeTemplate].zero)(_ |+| _)

We now know this is silly and we should have written:

  scala> templates.toIList.fold
  res: TradeTemplate = TradeTemplate(

.fold doesn’t work on stdlib List because it already has a method called fold that does it is own thing in its own special way.

The strangely named intercalate inserts a specific A between each element before performing the fold

  def intercalate[A: Monoid](fa: F[A], a: A): A = ...

which is a generalised version of the stdlib’s mkString:

  scala> List("foo", "bar").intercalate(",")
  res: String = "foo,bar"

The foldLeft provides the means to obtain any element by traversal index, including a bunch of other related methods:

  def index[A](fa: F[A], i: Int): Option[A] = ...
  def indexOr[A](fa: F[A], default: =>A, i: Int): A = ...
  def length[A](fa: F[A]): Int = ...
  def count[A](fa: F[A]): Int = length(fa)
  def empty[A](fa: F[A]): Boolean = ...
  def element[A: Equal](fa: F[A], a: A): Boolean = ...

Scalaz is a pure library of only total functions, whereas the stdlib .apply returns A and can throw an exception, Foldable.index returns an Option[A] with the convenient .indexOr returning an A when a default value is provided. .element is similar to the stdlib .contains but uses Equal rather than ill-defined JVM equality.

These methods really sound like a collections API. And, of course, anything with a Foldable can be converted into a List

  def toList[A](fa: F[A]): List[A] = ...

There are also conversions to other stdlib and scalaz data types such as .toSet, .toVector, .toStream, .to[T <: TraversableLike], .toIList and so on.

There are useful predicate checks

  def filterLength[A](fa: F[A])(f: A => Boolean): Int = ...
  def all[A](fa: F[A])(p: A => Boolean): Boolean = ...
  def any[A](fa: F[A])(p: A => Boolean): Boolean = ...

filterLength is a way of counting how many elements are true for a predicate, all and any return true if all (or any) element meets the predicate, and may exit early.

We can split an F[A] into parts that result in the same B with splitBy

  def splitBy[A, B: Equal](fa: F[A])(f: A => B): IList[(B, Nel[A])] = ...
  def splitByRelation[A](fa: F[A])(r: (A, A) => Boolean): IList[Nel[A]] = ...
  def splitWith[A](fa: F[A])(p: A => Boolean): List[Nel[A]] = ...
  def selectSplit[A](fa: F[A])(p: A => Boolean): List[Nel[A]] = ...
  def findLeft[A](fa: F[A])(f: A => Boolean): Option[A] = ...
  def findRight[A](fa: F[A])(f: A => Boolean): Option[A] = ...

for example

  scala> IList("foo", "bar", "bar", "faz", "gaz", "baz").splitBy(_.charAt(0))
  res = [(f, [foo]), (b, [bar, bar]), (f, [faz]), (g, [gaz]), (b, [baz])]

noting that there are two parts indexed by f.

splitByRelation avoids the need for an Equal but we must provide the comparison operator.

splitWith splits the elements into groups that alternatively satisfy and don’t satisfy the predicate. selectSplit selects groups of elements that satisfy the predicate, discarding others. This is one of those rare occasions when two methods share the same type signature but have different meanings.

findLeft and findRight are for extracting the first element (from the left, or right, respectively) that matches a predicate.

Making further use of Equal and Order, we have the distinct methods which return groupings.

  def distinct[A: Order](fa: F[A]): IList[A] = ...
  def distinctE[A: Equal](fa: F[A]): IList[A] = ...
  def distinctBy[A, B: Equal](fa: F[A])(f: A => B): IList[A] =

distinct is implemented more efficiently than distinctE because it can make use of ordering and therefore use a quicksort-esque algorithm that is much faster than the stdlib’s naive List.distinct. Data structures (such as sets) can implement distinct in their Foldable without doing any work.

distinctBy allows grouping by the result of applying a function to the elements. For example, grouping names by their first letter.

We can make further use of Order by extracting the minimum or maximum element (or both extrema) including variations using the Of or By pattern to first map to another type or to use a different type to do the order comparison.

  def maximum[A: Order](fa: F[A]): Option[A] = ...
  def maximumOf[A, B: Order](fa: F[A])(f: A => B): Option[B] = ...
  def maximumBy[A, B: Order](fa: F[A])(f: A => B): Option[A] = ...
  def minimum[A: Order](fa: F[A]): Option[A] = ...
  def minimumOf[A, B: Order](fa: F[A])(f: A => B): Option[B] = ...
  def minimumBy[A, B: Order](fa: F[A])(f: A => B): Option[A] = ...
  def extrema[A: Order](fa: F[A]): Option[(A, A)] = ...
  def extremaOf[A, B: Order](fa: F[A])(f: A => B): Option[(B, B)] = ...
  def extremaBy[A, B: Order](fa: F[A])(f: A => B): Option[(A, A)] =

For example we can ask which String is maximum By length, or what is the maximum length Of the elements.

  scala> List("foo", "fazz").maximumBy(_.length)
  res: Option[String] = Some(fazz)
  scala> List("foo", "fazz").maximumOf(_.length)
  res: Option[Int] = Some(4)

This concludes the key features of Foldable. You are forgiven for already forgetting all the methods you’ve just seen: the key takeaway is that anything you’d expect to find in a collection library is probably on Foldable and if it isn’t already, it probably should be.

We’ll conclude with some variations of the methods we’ve already seen. First there are methods that take a Semigroup instead of a Monoid:

  def fold1Opt[A: Semigroup](fa: F[A]): Option[A] = ...
  def foldMap1Opt[A, B: Semigroup](fa: F[A])(f: A => B): Option[B] = ...
  def sumr1Opt[A: Semigroup](fa: F[A]): Option[A] = ...
  def suml1Opt[A: Semigroup](fa: F[A]): Option[A] = ...

returning Option to account for empty data structures (recall that Semigroup does not have a zero).

The typeclass Foldable1 contains a lot more Semigroup variants of the Monoid methods shown here (all suffixed 1) and makes sense for data structures which are never empty, without requiring a Monoid on the elements.

Very importantly, there are variants that take monadic return values. We already used foldLeftM when we first wrote the business logic of our application, now you know that Foldable is where it came from:

  def foldLeftM[G[_]: Monad, A, B](fa: F[A], z: B)(f: (B, A) => G[B]): G[B] = ...
  def foldRightM[G[_]: Monad, A, B](fa: F[A], z: =>B)(f: (A, =>B) => G[B]): G[B] = ...
  def foldMapM[G[_]: Monad, A, B: Monoid](fa: F[A])(f: A => G[B]): G[B] = ...
  def findMapM[M[_]: Monad, A, B](fa: F[A])(f: A => M[Option[B]]): M[Option[B]] = ...
  def allM[G[_]: Monad, A](fa: F[A])(p: A => G[Boolean]): G[Boolean] = ...
  def anyM[G[_]: Monad, A](fa: F[A])(p: A => G[Boolean]): G[Boolean] = ...

You may also see Curried versions, e.g.

  def foldl[A, B](fa: F[A], z: B)(f: B => A => B): B = ...
  def foldr[A, B](fa: F[A], z: =>B)(f: A => (=> B) => B): B = ...

5.4.3 Traverse

Traverse is what happens when you cross a Functor with a Foldable

  trait Traverse[F[_]] extends Functor[F] with Foldable[F] {
    def traverse[G[_]: Applicative, A, B](fa: F[A])(f: A => G[B]): G[F[B]]
    def sequence[G[_]: Applicative, A](fga: F[G[A]]): G[F[A]] = ...
    def reverse[A](fa: F[A]): F[A] = ...
    def zipL[A, B](fa: F[A], fb: F[B]): F[(A, Option[B])] = ...
    def zipR[A, B](fa: F[A], fb: F[B]): F[(Option[A], B)] = ...
    def indexed[A](fa: F[A]): F[(Int, A)] = ...
    def zipWithL[A, B, C](fa: F[A], fb: F[B])(f: (A, Option[B]) => C): F[C] = ...
    def zipWithR[A, B, C](fa: F[A], fb: F[B])(f: (Option[A], B) => C): F[C] = ...
    def mapAccumL[S, A, B](fa: F[A], z: S)(f: (S, A) => (S, B)): (S, F[B]) = ...
    def mapAccumR[S, A, B](fa: F[A], z: S)(f: (S, A) => (S, B)): (S, F[B]) = ...

At the beginning of the chapter we showed the importance of traverse and sequence for swapping around type constructors to fit a requirement (e.g. List[Future[_]] to Future[List[_]]). You will use these methods more than you could possibly imagine.

In Foldable we weren’t able to assume that reverse was a universal concept, but now we can reverse a thing.

We can also zip together two things that have a Traverse, getting back None when one side runs out of elements, using zipL or zipR to decide which side to truncate when the lengths don’t match. A special case of zip is to add an index to every entry with indexed.

zipWithL and zipWithR allow combining the two sides of a zip into a new type, and then returning just an F[C].

mapAccumL and mapAccumR are regular map combined with an accumulator. If you find your old Java sins are making you want to reach for a var, and refer to it from a map, you want mapAccumL.

For example, let’s say we have a list of words and we want to blank out words we’ve already seen. The filtering algorithm is not allowed to process the list of words a second time so it can be scaled to an infinite stream:

  scala> val freedom =
  """We campaign for these freedoms because everyone deserves them.
     With these freedoms, the users (both individually and collectively)
     control the program and what it does for them."""
  scala> def clean(s: String): String = s.toLowerCase.replaceAll("[,.()]+", "")
  scala> freedom
         .mapAccumL(Set.empty[String]) { (seen, word) =>
           val cleaned = clean(word)
           (seen + cleaned, if (seen(cleaned)) "_" else word)
         .intercalate(" ")
  res: String =
  """We campaign for these freedoms because everyone deserves them.
     With _ _ the users (both individually and collectively)
     control _ program _ what it does _ _"""

Finally Traverse1, like Foldable1, provides variants of these methods for data structures that cannot be empty, accepting the weaker Semigroup instead of a Monoid, and an Apply instead of an Applicative.

5.4.4 Align

Align is about merging and padding anything with a Functor. Before looking at Align, meet the \&/ data type (spoken as These, or hurray!).

  sealed abstract class \&/[+A, +B]
  final case class This[A](aa: A) extends (A \&/ Nothing)
  final case class That[B](bb: B) extends (Nothing \&/ B)
  final case class Both[A, B](aa: A, bb: B) extends (A \&/ B)

i.e. it is a data encoding of inclusive logical OR.

  @typeclass trait Align[F[_]] extends Functor[F] {
    def alignWith[A, B, C](f: A \&/ B => C): (F[A], F[B]) => F[C]
    def align[A, B](a: F[A], b: F[B]): F[A \&/ B] = ...
    def merge[A: Semigroup](a1: F[A], a2: F[A]): F[A] = ...
    def pad[A, B]: (F[A], F[B]) => F[(Option[A], Option[B])] = ...
    def padWith[A, B, C](f: (Option[A], Option[B]) => C): (F[A], F[B]) => F[C] = ...

Hopefully by this point you are becoming more capable of reading type signatures to understand the purpose of a method.

alignWith takes a function from either an A or a B (or both) to a C and returns a lifted function from a tuple of F[A] and F[B] to an F[C]. align constructs a \&/ out of two F[_].

merge allows us to combine two F[A] when A has a Semigroup. For example, the implementation of Semigroup[Map[K, V]] defers to Semigroup[V], combining two entries results in combining their values, having the consequence that Map[K, List[A]] behaves like a multimap:

  scala> Map("foo" -> List(1)) merge Map("foo" -> List(1), "bar" -> List(2))
  res = Map(foo -> List(1, 1), bar -> List(2))

and a Map[K, Int] simply tally their contents when merging:

  scala> Map("foo" -> 1) merge Map("foo" -> 1, "bar" -> 2)
  res = Map(foo -> 2, bar -> 2)

.pad and .padWith are for partially merging two data structures that might be missing values on one side. For example if we wanted to aggregate independent votes and retain the knowledge of where the votes came from

  scala> Map("foo" -> 1) pad Map("foo" -> 1, "bar" -> 2)
  res = Map(foo -> (Some(1),Some(1)), bar -> (None,Some(2)))
  scala> Map("foo" -> 1, "bar" -> 2) pad Map("foo" -> 1)
  res = Map(foo -> (Some(1),Some(1)), bar -> (Some(2),None))

There are convenient variants of align that make use of the structure of \&/

    def alignSwap[A, B](a: F[A], b: F[B]): F[B \&/ A] = ...
    def alignA[A, B](a: F[A], b: F[B]): F[Option[A]] = ...
    def alignB[A, B](a: F[A], b: F[B]): F[Option[B]] = ...
    def alignThis[A, B](a: F[A], b: F[B]): F[Option[A]] = ...
    def alignThat[A, B](a: F[A], b: F[B]): F[Option[B]] = ...
    def alignBoth[A, B](a: F[A], b: F[B]): F[Option[(A, B)]] = ...

which should make sense from their type signatures. Examples:

  scala> List(1,2,3) alignSwap List(4,5)
  res = List(Both(4,1), Both(5,2), That(3))
  scala> List(1,2,3) alignA List(4,5)
  res = List(Some(1), Some(2), Some(3))
  scala> List(1,2,3) alignB List(4,5)
  res = List(Some(4), Some(5), None)
  scala> List(1,2,3) alignThis List(4,5)
  res = List(None, None, Some(3))
  scala> List(1,2,3) alignThat List(4,5)
  res = List(None, None, None)
  scala> List(1,2,3) alignBoth List(4,5)
  res = List(Some((1,4)), Some((2,5)), None)

Note that the A and B variants use inclusive OR, whereas the This and That variants are exclusive, returning None if there is a value in both sides, or no value on either side.

5.5 Variance

We must return to Functor for a moment and discuss an ancestor that we previously ignored:

InvariantFunctor, also known as the exponential functor, has a method xmap which says that given a function from A to B, and a function from B to A, then we can convert F[A] to F[B].

Functor is a short name for what should be covariant functor. But since Functor is so popular it gets the nickname. Likewise Contravariant should really be contravariant functor.

Functor implements xmap with map and ignores the function from B to A. Contravariant, on the other hand, implements xmap with contramap and ignores the function from A to B:

  @typeclass trait InvariantFunctor[F[_]] {
    def xmap[A, B](fa: F[A], f: A => B, g: B => A): F[B]
  @typeclass trait Functor[F[_]] extends InvariantFunctor[F] {
    def map[A, B](fa: F[A])(f: A => B): F[B]
    def xmap[A, B](fa: F[A], f: A => B, g: B => A): F[B] = map(fa)(f)
  @typeclass trait Contravariant[F[_]] extends InvariantFunctor[F] {
    def contramap[A, B](fa: F[A])(f: B => A): F[B]
    def xmap[A, B](fa: F[A], f: A => B, g: B => A): F[B] = contramap(fa)(g)

It is important to note that, although related at a theoretical level, the words covariant, contravariant and invariant do not directly refer to Scala type variance (i.e. + and - prefixes that may be written in type signatures). Invariance here means that it is possible to map the contents of a structure F[A] into F[B]. Using identity we can see that A can be safely downcast (or upcast) into B depending on the variance of the functor.

This sounds so hopelessly abstract that it needs a practical example immediately, before we can take it seriously. In Chapter 4 we used spray-json-derivation to derive a JSON encoder for our data types and we gave a brief description of the JsonWriter typeclass. This is an expanded version:

  @typeclass trait JsonWriter[A] { self =>
    def toJson(a: A): JsValue
    def contramap[B](f: B => A): JsonWriter[B] = new JsonWriter[B] {
      def toJson(b: B): JsValue = self(f(b))

Now consider the case where we want to write an instance of an JsonWriter[B] in terms of another JsonWriter[A], for example if we have a data type Alpha that simply wraps a Double. This is exactly what contramap is for:

  final case class Alpha(value: Double)
  object Alpha {
    implicit val encoder: JsonWriter[Alpha] = JsonWriter[Double].contramap(_.value)

On the other hand, a JsonReader typically has a Functor:

  @typeclass trait JsonReader[A] { self =>
    def fromJson(j: JsValue): JsonReader.Result[A]
    def map[B](f: A => B): JsonReader[B] = new JsonReader[B] {
      def fromJson(j: JsValue): B = f(self.fromJson(j))

Methods on a typeclass can have their type parameters in contravariant position (method parameters) or in covariant position (return type). If a typeclass has a combination of covariant and contravariant positions, it might have an invariant functor.

Consider what happens if we combine JsonWriter and JsonReader into one typeclass. We can no longer construct a Format by using map or contramap alone, we need xmap:

  @typeclass trait JsonFormat[A] extends JsonWriter[A] with JsonReader[A] { self =>
    def xmap[B](f: A => B, g: B => A): JsonFormat[B] = new JsonFormat[B] {
      def toJson(b: B): JsValue = self(g(b))
      def fromJson(j: JsValue): B = f(self.fromJson(j))

One of the most compelling uses for xmap is to provide typeclasses for value types. A value type is a compiletime wrapper for another type, that does not incur any object allocation costs (subject to some rules of use).

For example we can provide context around some numbers to avoid getting them mixed up:

  final case class Alpha(value: Double) extends AnyVal
  final case class Beta (value: Double) extends AnyVal
  final case class Rho  (value: Double) extends AnyVal
  final case class Nu   (value: Double) extends AnyVal

If we want to put these types in a JSON message, we’d need to write a custom Format for each type, which is tedious. But our Format implements xmap, allowing Format to be constructed from a simple pattern:

  implicit val double: JsonFormat[Double] = ...
  implicit val alpha: JsonFormat[Alpha] = double.xmap(Alpha(_), _.value)
  implicit val beta : JsonFormat[Beta]  = double.xmap(Beta(_) , _.value)
  implicit val rho  : JsonFormat[Rho]   = double.xmap(Rho(_)  , _.value)
  implicit val nu   : JsonFormat[Nu]    = double.xmap(Nu(_)   , _.value)

Macros can automate the construction of these instances, so we don’t need to write them: we’ll revisit this later in a dedicated chapter on Typeclass Derivation.

5.5.1 Composition

Invariants can be composed via methods with intimidating type signatures. There are many permutations of compose on most typeclasses, we will not list them all.

  @typeclass trait Functor[F[_]] extends InvariantFunctor[F] {
    def compose[G[_]: Functor]: Functor[λ[α => F[G[α]]]] = ...
    def icompose[G[_]: Contravariant]: Contravariant[λ[α => F[G[α]]]] = ...
  @typeclass trait Contravariant[F[_]] extends InvariantFunctor[F] {
    def compose[G[_]: Contravariant]: Functor[λ[α => F[G[α]]]] = ...
    def icompose[G[_]: Functor]: Contravariant[λ[α => F[G[α]]]] = ...

The α => type syntax is a kind-projector type lambda that says if Functor[F] is composed with a type G[_] (that has a Functor[G]), we get a Functor[F[G[_]]] that operates on the A in F[G[A]].

An example of Functor.compose is where F[_] is List, G[_] is Option, and we want to be able to map over the Int inside a List[Option[Int]] without changing the two structures:

  scala> val lo = List(Some(1), None, Some(2))
  scala> Functor[List].compose[Option].map(lo)(_ + 1)
  res: List[Option[Int]] = List(Some(2), None, Some(3))

This lets us jump into nested effects and structures and apply a function at the layer we want.

5.6 Apply and Bind

Consider this the warm-up act to Applicative and Monad

5.6.1 Apply

Apply extends Functor by adding a method named ap which is similar to map in that it applies a function to values. However, with ap, the function is in a similar context to the values.

  @typeclass trait Apply[F[_]] extends Functor[F] {
    @op("<*>") def ap[A, B](fa: =>F[A])(f: =>F[A => B]): F[B]

It is worth taking a moment to consider what that means for a simple data structure like Option[A], having the following implementation of .ap

  implicit def option[A]: Apply[Option[A]] = new Apply[Option[A]] {
    override def ap[A, B](fa: =>Option[A])(f: =>Option[A => B]) = f match {
      case Some(ff) =>
      case None    => None

To implement .ap, we must first extract the function ff: A => B from f: Option[A => B], then we can map over fa. The extraction of the function from the context is the important power that Apply brings, allowing multiple function to be combined inside the context.

Returning to Apply, we find .applyX boilerplate that allows us to combine parallel functions and then map over their combined output:

  @typeclass trait Apply[F[_]] extends Functor[F] {
    def apply2[A,B,C](fa: =>F[A], fb: =>F[B])(f: (A, B) => C): F[C] = ...
    def apply3[A,B,C,D](fa: =>F[A],fb: =>F[B],fc: =>F[C])(f: (A,B,C) =>D): F[D] = ...
    def apply12[...]

Read .apply2 as a contract promising: “if you give me an F of A and an F of B, with a way of combining A and B into a C, then I can give you an F of C”. There are many uses for this contract and the two most important are:

  • constructing some typeclasses for a product type C from its constituents A and B
  • performing effects in parallel, like the drone and google algebras we created in Chapter 3, and then combining their results.

Indeed, Apply is so useful that it has special syntax:

  implicit class ApplyOps[F[_]: Apply, A](self: F[A]) {
    def *>[B](fb: F[B]): F[B] = Apply[F].apply2(self,fb)((_,b) => b)
    def <*[B](fb: F[B]): F[A] = Apply[F].apply2(self,fb)((a,_) => a)
    def |@|[B](fb: F[B]): ApplicativeBuilder[F, A, B] = ...
  class ApplicativeBuilder[F[_]: Apply, A, B](a: F[A], b: F[B]) {
    def tupled: F[(A, B)] = Apply[F].apply2(a, b)(Tuple2(_))
    def |@|[C](cc: F[C]): ApplicativeBuilder3[C] = ...
    sealed abstract class ApplicativeBuilder3[C](c: F[C]) {

which is exactly what we used in Chapter 3:

  (d.getBacklog |@| d.getAgents |@| m.getManaged |@| m.getAlive |@| m.getTime)

The syntax *> and <* offer a convenient way to ignore the output from one of two parallel effects.

Unfortunately, although the |@| syntax is clear, there is a problem in that a new ApplicativeBuilder object is allocated for each additional effect. If the work is I/O-bound, the memory allocation cost is insignificant. However, when performing CPU-bound work, use the alternative lifting with arity syntax, which does not produce any intermediate objects:

  def ^[F[_]: Apply,A,B,C](fa: =>F[A],fb: =>F[B])(f: (A,B) =>C): F[C] = ...
  def ^^[F[_]: Apply,A,B,C,D](fa: =>F[A],fb: =>F[B],fc: =>F[C])(f: (A,B,C) =>D): F[D] = ...
  def ^^^^^^[F[_]: Apply, ...]

used like

  ^^^^(d.getBacklog, d.getAgents, m.getManaged, m.getAlive, m.getTime)

or directly call applyX

  Apply[F].apply5(d.getBacklog, d.getAgents, m.getManaged, m.getAlive, m.getTime)

Despite being of most value for dealing with effects, Apply provides convenient syntax for dealing with data structures. Consider rewriting

  for {
    foo <- Option[String]
    bar <- Option[Int]
  } yield foo + bar.shows


  ( |@| + _.shows) : Option[String]

If we only want the combined output as a tuple, methods exist to do just that:

  @op("tuple") def tuple2[A,B](fa: =>F[A],fb: =>F[B]): F[(A,B)] = ...
  def tuple3[A,B,C](fa: =>F[A],fb: =>F[B],fc: =>F[C]): F[(A,B,C)] = ...
  def tuple12[...]
  ( tuple : Option[(String, Int)]

There are also the generalised versions of ap for more than two parameters:

  def ap2[A,B,C](fa: =>F[A],fb: =>F[B])(f: F[(A,B) => C]): F[C] = ...
  def ap3[A,B,C,D](fa: =>F[A],fb: =>F[B],fc: =>F[C])(f: F[(A,B,C) => D]): F[D] = ...
  def ap12[...]

along with lift methods that take normal functions and lift them into the F[_] context, the generalisation of Functor.lift

  def lift2[A,B,C](f: (A,B) => C): (F[A],F[B]) => F[C] = ...
  def lift3[A,B,C,D](f: (A,B,C) => D): (F[A],F[B],F[C]) => F[D] = ...
  def lift12[...]

and apF, a partially applied syntax for ap

  def apF[A,B](f: =>F[A => B]): F[A] => F[B] = ...

Finally forever

  def forever[A, B](fa: F[A]): F[B] = ...

repeating an effect without stopping. The instance of Apply must be stack safe or we’ll get StackOverflowError.

5.6.2 Bind

Bind introduces bind, synonymous with flatMap, which allows functions over the result of an effect to return a new effect, or for functions over the values of a data structure to return new data structures that are then joined.

  @typeclass trait Bind[F[_]] extends Apply[F] {
    @op(">>=") def bind[A, B](fa: F[A])(f: A => F[B]): F[B]
    def flatMap[A, B](fa: F[A])(f: A => F[B]): F[B] = bind(fa)(f)
    def join[A](ffa: F[F[A]]): F[A] = bind(ffa)(identity)
    def mproduct[A, B](fa: F[A])(f: A => F[B]): F[(A, B)] = ...
    def ifM[B](value: F[Boolean], t: =>F[B], f: =>F[B]): F[B] = ...

The .join may be familiar if you have ever used .flatten in the stdlib, it takes a nested context and squashes it into one.

Although not necessarily implemented as such, we can think of .bind as being a followed by .join

  def bind[A, B](fa: F[A])(f: A => F[B]): F[B] = join(map(fa)(f))

mproduct is like Functor.fproduct and pairs the function’s input with its output, inside the F.

ifM is a way to construct a conditional data structure or effect:

  scala> List(true, false, true).ifM(List(0), List(1, 1))
  res: List[Int] = List(0, 1, 1, 0)

ifM and ap are optimised to cache and reuse code branches, compare to the longer form

  scala> List(true, false, true).flatMap { b => if (b) List(0) else List(1, 1) }

which produces a fresh List(0) or List(1, 1) every time the branch is invoked.

Bind also has some special syntax

  implicit class BindOps[F[_]: Bind, A] (self: F[A]) {
    def >>[B](b: =>F[B]): F[B] = Bind[F].bind(self)(_ => b)
    def >>![B](f: A => F[B]): F[A] = Bind[F].bind(self)(a => f(a).map(_ => a))

>> is when we wish to discard the input to bind and >>! is when we want to run an effect but discard its output.

5.7 Applicative and Monad

From a functionality point of view, Applicative is Apply with a pure method, and Monad extends Applicative with Bind.

  @typeclass trait Applicative[F[_]] extends Apply[F] {
    def point[A](a: =>A): F[A]
    def pure[A](a: =>A): F[A] = point(a)
  @typeclass trait Monad[F[_]] extends Applicative[F] with Bind[F]

In many ways, Applicative and Monad are the culmination of everything we’ve seen in this chapter. .pure (or .point as it is more commonly known for data structures) allows us to create effects or data structures from values.

Instances of Applicative must meet some laws, effectively asserting that all the methods are consistent:

  • Identity: fa <*> pure(identity) === fa, (where fa is an F[A]) i.e. applying pure(identity) does nothing.
  • Homomorphism: pure(a) <*> pure(ab) === pure(ab(a)) (where ab is an A => B), i.e. applying a pure function to a pure value is the same as applying the function to the value and then using pure on the result.
  • Interchange: pure(a) <*> fab === fab <*> pure(f => f(a)), (where fab is an F[A => B]), i.e. pure is a left and right identity
  • Mappy: map(fa)(f) === fa <*> pure(f)

Monad adds additional laws:

  • Left Identity: pure(a).bind(f) === f(a)
  • Right Identity: a.bind(pure(_)) === a
  • Associativity: fa.bind(f).bind(g) === fa.bind(a => f(a).bind(g)) where fa is an F[A], f is an A => F[B] and g is a B => F[C].

Associativity says that chained bind calls must agree with nested bind. However, it does not mean that we can rearrange the order, which would be commutativity. For example, recalling that flatMap is an alias to bind, we cannot rearrange

  for {
    _ <- machine.start(node1)
    _ <- machine.stop(node1)
  } yield true


  for {
    _ <- machine.stop(node1)
    _ <- machine.start(node1)
  } yield true

start and stop are non-commutative, because the intended effect of starting then stopping a node is different to stopping then starting it!

But start is commutative with itself, and stop is commutative with itself, so we can rewrite

  for {
    _ <- machine.start(node1)
    _ <- machine.start(node2)
  } yield true


  for {
    _ <- machine.start(node2)
    _ <- machine.start(node1)
  } yield true

which are equivalent. We’re making a lot of assumptions about the Google Container API here, but this is a reasonable choice to make.

A practical consequence is that a Monad must be commutative if its applyX methods can be allowed to run in parallel. We cheated in Chapter 3 when we ran these effects in parallel

  (d.getBacklog |@| d.getAgents |@| m.getManaged |@| m.getAlive |@| m.getTime)

because we know that they are commutative among themselves. When it comes to interpreting our application, later in the book, we will have to provide evidence that these effects are in fact commutative, or an asynchronous implementation may choose to sequence the operations to be on the safe side.

The subtleties of how we deal with (re)-ordering of effects, and what those effects are, deserves a dedicated chapter on Advanced Monads.

5.8 Divide and Conquer

Divide is the Contravariant analogue of Apply

  @typeclass trait Divide[F[_]] extends Contravariant[F] {
    def divide[A, B, C](fa: F[A], fb: F[B])(f: C => (A, B)): F[C] = divide2(fa, fb)(f)
    def divide1[A1, Z](a1: F[A1])(f: Z => A1): F[Z] = ...
    def divide2[A, B, C](fa: F[A], fb: F[B])(f: C => (A, B)): F[C] = ...
    def divide22[...] = ...

divide says that if we can break a C into an A and a B, and we’re given an F[A] and an F[B], then we can get an F[C]. Hence, divide and conquer.

This is a great way to generate contravariant typeclass instances for product types by breaking the products into their parts. Scalaz has an instance of Divide[Equal], let’s construct an Equal for a new product type Foo

  scala> case class Foo(s: String, i: Int)
  scala> implicit val fooEqual: Equal[Foo] =
           Divide[Equal].divide2(Equal[String], Equal[Int]) {
             (foo: Foo) => (foo.s, foo.i)
  scala> Foo("foo", 1) === Foo("bar", 1)
  res: Boolean = false

It is a good moment to look again at Apply

  @typeclass trait Apply[F[_]] extends Functor[F] {
    def apply2[A, B, C](fa: =>F[A], fb: =>F[B])(f: (A, B) => C): F[C] = ...
    def apply3[A,B,C,D](fa: =>F[A],fb: =>F[B],fc: =>F[C])(f: (A,B,C) =>D): F[D] = ...
    def apply12[...]

It is now easier to spot that applyX is how we can derive typeclasses for covariant typeclasses.

Mirroring Apply, Divide also has terse syntax for tuples. A softer divide so that you may reign approach to world domination:

    def tuple2[A1, A2](a1: F[A1], a2: F[A2]): F[(A1, A2)] = ...
    def tuple22[...] = ...
    def deriving2[A1: F, A2: F, Z](f: Z => (A1, A2)): F[Z] = ...
    def deriving22[...] = ...

and deriving, which is even more convenient to use for typeclass derivation:

  implicit val fooEqual: Equal[Foo] = Divide[Equal].deriving2(f => (f.s, f.i))

Generally, if encoder typeclasses can provide an instance of Divide, rather than stopping at Contravariant, it makes it possible to derive instances for any case class. Similarly, decoder typeclasses can provide an Apply instance. We will explore this in a dedicated chapter on Typeclass Derivation.

Divisible is the Contravariant analogue of Applicative and introduces conquer, the equivalent of pure

  @typeclass trait Divisible[F[_]] extends Divide[F] {
    def conquer[A]: F[A]

conquer allows creating fallback implementations that effectively ignore the type parameter. Such values are called universally quantified. For example, the Divisible[Equal].conquer[String] returns a trivial implementation of Equal that always returns true, which allows us to implement contramap in terms of divide

  override def contramap[A, B](fa: F[A])(f: B => A): F[B] =
    divide(conquer[Unit], fa)(c => ((), f(c)))

5.9 Plus

Plus is Semigroup but for type constructors, and PlusEmpty is the equivalent of Monoid (they even have the same laws) whereas IsEmpty is novel and allows us to query if an F[A] is empty:

  @typeclass trait Plus[F[_]] {
    @op("<+>") def plus[A](a: F[A], b: =>F[A]): F[A]
  @typeclass trait PlusEmpty[F[_]] extends Plus[F] {
    def empty[A]: F[A]
  @typeclass trait IsEmpty[F[_]] extends PlusEmpty[F] {
    def isEmpty[A](fa: F[A]): Boolean

Although it may look on the surface as if <+> behaves like |+|

  scala> List(2,3) |+| List(7)
  res = List(2, 3, 7)
  scala> List(2,3) <+> List(7)
  res = List(2, 3, 7)

it is best to think of it as operating only at the F[_] level, never looking into the contents. Plus has the convention that it should ignore failures and “pick the first winner”. <+> can therefore be used as a mechanism for early exit (losing information) and failure-handling via fallbacks:

  scala> Option(1) |+| Option(2)
  res = Some(3)
  scala> Option(1) <+> Option(2)
  res = Some(1)
  scala> Option.empty[Int] <+> Option(1)
  res = Some(1)

For example, if we have a NonEmptyList[Option[Int]] and we want to ignore None values (failures) and pick the first winner (Some), we can call <+> from Foldable1.foldRight1:

  scala> NonEmptyList(None, None, Some(1), Some(2), None)
         .foldRight1(_ <+> _)
  res: Option[Int] = Some(1)

In fact, now that we know about Plus, we release that we didn’t need to break typeclass coherence (when we defined a locally scoped Monoid[Option[A]]) in the section on Appendable Things. Our objective was to “pick the last winner”, which is the same as “pick the winner” if the arguments are swapped. Note the use of the TIE Interceptor for ccy and otc and that b comes before a in

  implicit val monoid: Monoid[TradeTemplate] = Monoid.instance(
    (a, b) =>
      TradeTemplate(a.payments |+| b.payments,
                    b.ccy <+> a.ccy,
                    b.otc <+> a.otc),
    TradeTemplate(Nil, None, None)

Applicative and Monad have specialised versions of PlusEmpty

  @typeclass trait ApplicativePlus[F[_]] extends Applicative[F] with PlusEmpty[F]
  @typeclass trait MonadPlus[F[_]] extends Monad[F] with ApplicativePlus[F] {
    def unite[T[_]: Foldable, A](ts: F[T[A]]): F[A] = ...
    def withFilter[A](fa: F[A])(f: A => Boolean): F[A] = ...

ApplicativePlus is also known as Alternative.

unite looks a Foldable.fold on the contents of F[_] but is folding with the PlusEmpty[F].monoid (not the Monoid[A]). For example, uniting List[Either[_, _]] means Left becomes empty (Nil) and the contents of Right become single element List, which are then concatenated:

  scala> List(Right(1), Left("boo"), Right(2)).unite
  res: List[Int] = List(1, 2)
  scala> val boo: Either[String, Int] = Left("boo")
         boo.foldMap(a => a.pure[List])
  res: List[String] = List()
  scala> val n: Either[String, Int] = Right(1)
         n.foldMap(a => a.pure[List])
  res: List[Int] = List(1)

withFilter allows us to make use of for comprehension language support as discussed in Chapter 2. It is fair to say that the Scala language has built-in language support for MonadPlus, not just Monad!

Returning to Foldable for a moment, we can reveal some methods that we did not discuss earlier

  @typeclass trait Foldable[F[_]] {
    def msuml[G[_]: PlusEmpty, A](fa: F[G[A]]): G[A] = ...
    def collapse[X[_]: ApplicativePlus, A](x: F[A]): X[A] = ...

msuml does a fold using the Monoid from the PlusEmpty[G] and collapse does a foldRight using the PlusEmpty of the target type:

  scala> IList(Option(1), Option.empty[Int], Option(2)).fold
  res: Option[Int] = Some(3) // uses Monoid[Option[Int]]
  scala> IList(Option(1), Option.empty[Int], Option(2)).msuml
  res: Option[Int] = Some(1) // uses PlusEmpty[Option].monoid
  scala> IList(1, 2).collapse[Option]
  res: Option[Int] = Some(1)

5.10 Lone Wolves

Some of the typeclasses in scalaz are stand-alone and not part of the larger hierarchy.

5.10.1 Zippy

  @typeclass trait Zip[F[_]]  {
    def zip[A, B](a: =>F[A], b: =>F[B]): F[(A, B)]
    def zipWith[A, B, C](fa: =>F[A], fb: =>F[B])(f: (A, B) => C)
                        (implicit F: Functor[F]): F[C] = ...
    def ap(implicit F: Functor[F]): Apply[F] = ...
    @op("<*|*>") def apzip[A, B](f: =>F[A] => F[B], a: =>F[A]): F[(A, B)] = ...

The core method is zip which is a less powerful version of Divide.tuple2, and if a Functor[F] is provided then zipWith can behave like Apply.apply2. Indeed, an Apply[F] can be created from a Zip[F] and a Functor[F] by calling ap.

apzip takes an F[A] and a lifted function from F[A] => F[B], producing an F[(A, B)] similar to Functor.fproduct.

  @typeclass trait Unzip[F[_]]  {
    @op("unfzip") def unzip[A, B](a: F[(A, B)]): (F[A], F[B])
    def firsts[A, B](a: F[(A, B)]): F[A] = ...
    def seconds[A, B](a: F[(A, B)]): F[B] = ...
    def unzip3[A, B, C](x: F[(A, (B, C))]): (F[A], F[B], F[C]) = ...
    def unzip7[A ... H](x: F[(A, (B, ... H))]): ...

The core method is unzip with firsts and seconds allowing for selecting either the first or second element of a tuple in the F. Importantly, unzip is the opposite of zip.

The methods unzip3 to unzip7 are repeated applications of unzip to save on boilerplate. For example, if handed a bunch of nested tuples, the Unzip[Id] is a handy way to flatten them:

  scala> Unzip[Id].unzip7((1, (2, (3, (4, (5, (6, 7)))))))
  res = (1,2,3,4,5,6,7)

In a nutshell, Zip and Unzip are less powerful versions of Divide and Apply, providing useful features without requiring the F to make too many promises.

5.10.2 Optional

Optional is a generalisation of data structures that can optionally contain a value, like Option and Either.

Recall that \/ (disjunction) is scalaz’s improvement of scala.Either. We will also see Maybe, scalaz’s improvement of scala.Option

  sealed abstract class Maybe[A]
  final case class Empty[A]()    extends Maybe[A]
  final case class Just[A](a: A) extends Maybe[A]
  @typeclass trait Optional[F[_]] {
    def pextract[B, A](fa: F[A]): F[B] \/ A
    def getOrElse[A](fa: F[A])(default: =>A): A = ...
    def orElse[A](fa: F[A])(alt: =>F[A]): F[A] = ...
    def isDefined[A](fa: F[A]): Boolean = ...
    def nonEmpty[A](fa: F[A]): Boolean = ...
    def isEmpty[A](fa: F[A]): Boolean = ...
    def toOption[A](fa: F[A]): Option[A] = ...
    def toMaybe[A](fa: F[A]): Maybe[A] = ...

These are methods that should be familiar, except perhaps pextract, which is a way of letting the F[_] return some implementation specific F[B] or the value. For example, Optional[Option].pextract returns Option[Nothing] \/ A, i.e. None \/ A.

Scalaz gives a ternary operator to things that have an Optional

  implicit class OptionalOps[F[_]: Optional, A](fa: F[A]) {
    def ?[X](some: =>X): Conditional[X] = new Conditional[X](some)
    final class Conditional[X](some: =>X) {
      def |(none: =>X): X = if (Optional[F].isDefined(fa)) some else none

for example

  scala> val knock_knock: Option[String] = ...
         knock_knock ? "who's there?" | "<tumbleweed>"

Next time you write a function that takes an Option, consider rewriting it to take Optional instead: it’ll make it easier to migrate to data structures that have better error handling without any loss of functionality.

5.10.3 Catchable

Our grand plans to write total functions that return a value for every input may be in ruins when exceptions are the norm in the Java standard library, the Scala standard library, and the myriad of legacy systems that we must interact with.

scalaz does not magically handle exceptions automatically, but it does provide the mechanism to protect against bad legacy systems.

  @typeclass trait Catchable[F[_]] {
    def attempt[A](f: F[A]): F[Throwable \/ A]
    def fail[A](err: Throwable): F[A]

attempt will catch any exceptions inside F[_] and make the JVM Throwable an explicit return type that can be mapped into an error reporting ADT, or left as an indicator to downstream callers that Here be Dragons.

fail permits callers to throw an exception in the F[_] context and, since this breaks purity, will be removed from scalaz. Exceptions that are raised via fail must be later handled by attempt since it is just as bad as calling legacy code that throws an exception.

It is worth noting that Catchable[Id] cannot be implemented. An Id[A] cannot exist in a state that may contain an exception. However, there are instances for both scala.concurrent.Future (asynchronous) and scala.Either (synchronous), allowing Catchable to abstract over the unhappy path. MonadError, as we will see in a later chapter, is a superior replacement.

5.11 Co-things

A co-thing typically has some opposite type signature to whatever thing does, but is not necessarily its inverse. To highlight the relationship between thing and co-thing, we will include the type signature of thing wherever we can.

5.11.1 Cobind

  @typeclass trait Cobind[F[_]] extends Functor[F] {
    def cobind[A, B](fa: F[A])(f: F[A] => B): F[B]
  //def   bind[A, B](fa: F[A])(f: A => F[B]): F[B]
    def cojoin[A](fa: F[A]): F[F[A]] = ...
  //def   join[A](ffa: F[F[A]]): F[A] = ...

cobind (also known as coflatmap) takes an F[A] => B that acts on an F[A] rather than its elements. But this is not necessarily the full fa, it is usually some substructure as defined by cojoin (also known as coflatten) which expands a data structure.

Compelling use-cases for Cobind are rare, although when shown in the Functor permutation table (for F[_], A and B) it is difficult to argue why any method should be less important than the others:

method parameter
map A => B
contramap B => A
xmap (A => B, B => A)
ap F[A => B]
bind A => F[B]
cobind F[A] => B

5.11.2 Comonad

  @typeclass trait Comonad[F[_]] extends Cobind[F] {
    def copoint[A](p: F[A]): A
  //def   point[A](a: =>A): F[A]

copoint (also copure) unwraps an element from a context. When interpreting a pure program, we typically require a Comonad to run the interpreter inside the application’s def main entry point. For example, Comonad[Future].copoint will await the execution of a Future[Unit].

Far more interesting is the Comonad of a data structure. This is a way to construct a view of all elements alongside their neighbours. Consider a neighbourhood (Hood for short) for a list containing all the elements to the left of an element (lefts), the element itself (the focus), and all the elements to its right (rights).

  final case class Hood[A](lefts: IList[A], focus: A, rights: IList[A])

The lefts and rights should each be ordered with the nearest to the focus at the head, such that we can recover the original IList via .toList

  object Hood {
    implicit class Ops[A](hood: Hood[A]) {
      def toList: IList[A] = hood.lefts.reverse ::: hood.focus :: hood.rights

We can write methods that let us move the focus one to the left (previous) and one to the right (next)

      def previous: Maybe[Hood[A]] = hood.lefts match {
        case INil() => Empty()
        case ICons(head, tail) =>
          Just(Hood(tail, head, hood.focus :: hood.rights))
      def next: Maybe[Hood[A]] = hood.rights match {
        case INil() => Empty()
        case ICons(head, tail) =>
          Just(Hood(hood.focus :: hood.lefts, head, tail))

By introducing more to repeatedly apply an optional function to Hood we can calculate all the positions that Hood can take in the list

      def more(f: Hood[A] => Maybe[Hood[A]]): IList[Hood[A]] =
        f(hood) match {
          case Empty() => INil()
          case Just(r) => ICons(r, r.more(f))
      def positions: Hood[Hood[A]] = {
        val left  = hood.more(_.previous)
        val right = hood.more(
        Hood(left, hood, right)

We can now implement Comonad[Hood]

    implicit val comonad: Comonad[Hood] = new Comonad[Hood] {
      def map[A, B](fa: Hood[A])(f: A => B): Hood[B] =
        Hood(, f(fa.focus),
      def cobind[A, B](fa: Hood[A])(f: Hood[A] => B): Hood[B] =
      def copoint[A](fa: Hood[A]): A = fa.focus

cojoin gives us a Hood[Hood[IList]] containing all the possible neighbourhoods in our initial IList

  scala> val middle = Hood(IList(4, 3, 2, 1), 5, IList(6, 7, 8, 9))
  res = Hood(

Indeed, cojoin is just positions! We can override it with a more direct (and performant) implementation

  override def cojoin[A](fa: Hood[A]): Hood[Hood[A]] = fa.positions

Comonad generalises the concept of Hood to arbitrary data structures. Hood is an example of a zipper (unrelated to Zip). Scalaz comes with a Zipper data type for streams (i.e. infinite 1D data structures), which we will discuss in the next chapter.

One application of a zipper is for cellular automata, which compute the value of each cell in the next generation by performing a computation based on the neighbourhood of that cell.

5.11.3 Cozip

  @typeclass trait Cozip[F[_]] {
    def cozip[A, B](x: F[A \/ B]): F[A] \/ F[B]
  //def   zip[A, B](a: =>F[A], b: =>F[B]): F[(A, B)]
  //def unzip[A, B](a: F[(A, B)]): (F[A], F[B])
    def cozip3[A, B, C](x: F[A \/ (B \/ C)]): F[A] \/ (F[B] \/ F[C]) = ...
    def cozip7[A ... H](x: F[(A \/ (... H))]): F[A] \/ (... F[H]) = ...

Although named cozip, it is perhaps more appropriate to talk about its symmetry with unzip. Whereas unzip splits F[_] of tuples (products) into tuples of F[_], cozip splits F[_] of disjunctions (coproducts) into disjunctions of F[_].

5.12 Bi-things

Sometimes we may find ourselves with a thing that has two type holes and we want to map over both sides. For example we might be tracking failures in the left of an Either and we want to do something with the failure messages.

The Functor / Foldable / Traverse typeclasses have bizarro relatives that allow us to map both ways.

  @typeclass trait Bifunctor[F[_, _]] {
    def bimap[A, B, C, D](fab: F[A, B])(f: A => C, g: B => D): F[C, D]
    @op("<-:") def leftMap[A, B, C](fab: F[A, B])(f: A => C): F[C, B] = ...
    @op(":->") def rightMap[A, B, D](fab: F[A, B])(g: B => D): F[A, D] = ...
    @op("<:>") def umap[A, B](faa: F[A, A])(f: A => B): F[B, B] = ...
  @typeclass trait Bifoldable[F[_, _]] {
    def bifoldMap[A, B, M: Monoid](fa: F[A, B])(f: A => M)(g: B => M): M
    def bifoldRight[A,B,C](fa: F[A, B], z: =>C)(f: (A, =>C) => C)(g: (B, =>C) => C): C
    def bifoldLeft[A,B,C](fa: F[A, B], z: C)(f: (C, A) => C)(g: (C, B) => C): C = ...
    def bifoldMap1[A, B, M: Semigroup](fa: F[A,B])(f: A => M)(g: B => M): Option[M] = ...
  @typeclass trait Bitraverse[F[_, _]] extends Bifunctor[F] with Bifoldable[F] {
    def bitraverse[G[_]: Applicative, A, B, C, D](fab: F[A, B])
                                                 (f: A => G[C])
                                                 (g: B => G[D]): G[F[C, D]]
    def bisequence[G[_]: Applicative, A, B](x: F[G[A], G[B]]): G[F[A, B]] = ...

Although the type signatures are verbose, these are nothing more than the core methods of Functor, Foldable and Bitraverse taking two functions instead of one, often requiring both functions to return the same type so that their results can be combined with a Monoid or Semigroup.

  scala> val a: Either[String, Int] = Left("fail")
         val b: Either[String, Int] = Right(13)
  scala> b.bimap(_.toUpperCase, _ * 2)
  res: Either[String, Int] = Right(26)
  scala> a.bimap(_.toUpperCase, _ * 2)
  res: Either[String, Int] = Left(FAIL)
  scala> b :-> (_ * 2)
  res: Either[String,Int] = Right(26)
  scala> a :-> (_ * 2)
  res: Either[String, Int] = Left(fail)
  scala> { s: String => s.length } <-: a
  res: Either[Int, Int] = Left(4)
  scala> a.bifoldMap(_.length)(identity)
  res: Int = 4
  scala> b.bitraverse(s => Future(s.length), i => Future(i))
  res: Future[Either[Int, Int]] = Future(<not completed>)

In addition, we can revisit MonadPlus (recall it is Monad with the ability to filterWith and unite) and see that it can separate Bifoldable contents of a Monad

  @typeclass trait MonadPlus[F[_]] {
    def separate[G[_, _]: Bifoldable, A, B](value: F[G[A, B]]): (F[A], F[B]) = ...

This is very useful if we have a collection of bi-things and we want to reorganise them into a collection of A and a collection of B

  scala> val list: List[Either[Int, String]] =
           List(Right("hello"), Left(1), Left(2), Right("world"))
  scala> list.separate
  res: (List[Int], List[String]) = (List(1, 2), List(hello, world))

5.13 Very Abstract Things

What remains of the typeclass hierarchy are things that allow us to meta-reason about functional programming and scalaz. We are not going to discuss these yet as they deserve a full chapter on Category Theory and are not needed in typical FP applications.

5.14 Summary

That was a lot of material! We have just explored a standard library of polymorphic functionality. But to put it into perspective: there are more traits in the Scala stdlib Collections API than typeclasses in scalaz.

It is normal for an FP application to only touch a small percentage of the typeclass hierarchy, with most functionality coming from domain-specific typeclasses. Even if the domain-specific typeclasses are just specialised clones of something in scalaz, it is better to write the code and later refactor it, than to over-abstract too early.

To help, we have included a cheat-sheet of the typeclasses and their primary methods in the Appendix, inspired by Adam Rosien’s Scalaz Cheatsheet. These cheat-sheets make an excellent replacement for the family portrait on your office desk.

To help further, Valentin Kasas explains how to combine N things:

6. Scalaz Data Types

Who doesn’t love a good data structure? The answer is nobody, because data structures are awesome.

In this chapter we’ll explore the collection-like data types in scalaz, as well as data types that augment the Scala language with useful semantics and additional type safety.

The primary reason we care about having lots of collections at our disposal is performance. A vector and a list can do the same things, but their performance characteristics are different: a vector has constant lookup cost whereas a list must be traversed.

All of the collections presented here are persistent: if we add or remove an element we can still use the old version. Structural sharing is essential to the performance of persistent data structures, otherwise the entire collection is rebuilt with every operation.

Unlike the Java and Scala collections, there is no hierarchy to the data types in scalaz: these collections are much simpler to understand. Polymorphic functionality is provided by optimised instances of the typeclasses we studied in the previous chapter. This makes it a lot easier to swap implementations for performance reasons, and to provide our own.

6.1 Type Variance

Many of scalaz’s data types are invariant in their type parameters. For example, IList[A] is not a subtype of IList[B] when A <: B.

6.1.1 Covariance

The problem with covariant type parameters, such as class List[+A], is that List[A] is a subtype of List[Any] and it is easy to accidentally lose type information.

  scala> List("hello") ++ List(' ') ++ List("world!")
  res: List[Any] = List(hello,  , world!)

Note that the second list is a List[Char] and the compiler has unhelpfully inferred the Least Upper Bound (LUB) to be Any. Compare to IList, which requires explicit .widen[Any] to permit the heinous crime:

  scala> IList("hello") ++ IList(' ') ++ IList("world!")
  <console>:35: error: type mismatch;
   found   : Char(' ')
   required: String
  scala> IList("hello").widen[Any]
           ++ IList(' ').widen[Any]
           ++ IList("world!").widen[Any]
  res: IList[Any] = [hello, ,world!]

Similarly, when the compiler infers a type with Product with Serializable it is a strong indicator that accidental widening has occurred due to covariance.

Unfortunately we must be careful when constructing invariant data types because LUB calculations are performed on the parameters:

  scala> IList("hello", ' ', "world")
  res: IList[Any] = [hello, ,world]

Another similar problem arises from Scala’s Nothing type, which is a subtype of all other types, including sealed ADTs, final classes, primitives and null.

There are no values of type Nothing: functions that take a Nothing as a parameter cannot be run and functions that return Nothing will never return. Nothing was introduced as a mechanism to enable covariant type parameters, but a consequence is that we can write un-runnable code, by accident. Scalaz says we do not need covariant type parameters which means that we are limiting ourselves to writing practical code that can be run.

To show how easy it is to introduce an inferred Nothing, the following code

  final case class Foos(things: List[String])

is inferred to be


Whereas an invariant list

  final case class Bars(v: IList[String])

is accurately inferred to be


with no need for a Nothing hack.

6.1.2 Contrarivariance

On the other hand, contravariant type parameters, such as trait Thing[-A], can expose devastating bugs in the compiler. Consider Paul Phillips’ (ex-scalac team) demonstration of what he calls contrarivariance:

  scala> :paste
         trait Thing[-A]
         def f(x: Thing[ Seq[Int]]): Byte   = 1
         def f(x: Thing[List[Int]]): Short  = 2
  scala> f(new Thing[ Seq[Int]] { })
         f(new Thing[List[Int]] { })
  res = 1
  res = 2

As expected, the compiler is finding the most specific argument in each call to f. However, implicit resolution gives unexpected results:

  scala> :paste
         implicit val t1: Thing[ Seq[Int]] =
           new Thing[ Seq[Int]] { override def toString = "1" }
         implicit val t2: Thing[List[Int]] =
           new Thing[List[Int]] { override def toString = "2" }
  scala> implicitly[Thing[ Seq[Int]]]
  res = 1
  res = 1

Implicit resolution flips its definition of “most specific” for contravariant types, rendering them useless for typeclasses or anything that requires polymorphic functionality.

6.1.3 Limitations of subtyping

scala.Option has a method .flatten which will convert Option[Option[B]] into an Option[B]. However, Scala’s type system is unable to let us write the required type signature. Consider the following that appears correct, but has a subtle bug:

  sealed abstract class Option[+A] {
    def flatten[B, A <: Option[B]]: Option[B] = ...

The A introduced on .flatten is shadowing the A introduced on the class. It is equivalent to writing

  sealed abstract class Option[+A] {
    def flatten[B, C <: Option[B]]: Option[B] = ...

which is not the constraint we want.

To workaround this limitation, Scala defines infix classes <:< and =:= along with implicit evidence that always creates a witness

  sealed abstract class <:<[-From, +To] extends (From => To)
  implicit def conforms[A]: A <:< A = new <:<[A, A] { def apply(x: A): A = x }
  sealed abstract class =:=[ From,  To] extends (From => To)
  implicit def tpEquals[A]: A =:= A = new =:=[A, A] { def apply(x: A): A = x }

=:= can be used to require that two type parameters are exactly the same and <:< is used to describe subtype relationships, letting us implement .flatten as

  sealed abstract class Option[+A] {
    def flatten[B](implicit ev: A <:< Option[B]): Option[B] = this match {
      case None        => None
      case Some(value) => ev(value)
  final case class Some[+A](value: A) extends Option[A] 
  case object None                    extends Option[Nothing]

Scalaz improves on <:< and =:= with Liskov (aliased to <~<) and Leibniz (===).

  sealed abstract class Liskov[-A, +B] {
    def apply(a: A): B = ...
    def subst[F[-_]](p: F[B]): F[A]
    def andThen[C](that: Liskov[B, C]): Liskov[A, C] = ...
    def onF[X](fa: X => A): X => B = ...
  object Liskov {
    type <~<[-A, +B] = Liskov[A, B]
    type >~>[+B, -A] = Liskov[A, B]
    implicit def refl[A]: (A <~< A) = ...
    implicit def isa[A, B >: A]: A <~< B = ...
    implicit def witness[A, B](lt: A <~< B): A => B = ...
  // type signatures have been simplified
  sealed abstract class Leibniz[A, B] {
    def apply(a: A): B = ...
    def subst[F[_]](p: F[A]): F[B]
    def flip: Leibniz[B, A] = ...
    def andThen[C](that: Leibniz[B, C]): Leibniz[A, C] = ...
    def onF[X](fa: X => A): X => B = ...
  object Leibniz {
    type ===[A, B] = Leibniz[A, B]
    implicit def refl[A]: Leibniz[A, A] = ...
    implicit def subst[A, B](a: A)(implicit f: A === B): B = ...
    implicit def witness[A, B](f: A === B): A => B = ...

Other than generally useful methods and implicit conversions, the scalaz <~< and === evidence is more principled than in the stdlib.

6.2 Evaluation

Java is a strict evaluation language: all the parameters to a method must be evaluated to a value before the method is called. Scala introduces the notion of by-name parameters on methods with a: =>A syntax. These parameters are wrapped up as a zero argument function which is called every time the a is referenced. We seen by-name a lot in the typeclasses.

Scala also has by-need evaluation of values, with the lazy keyword: the computation is evaluated at most once to produce the value. Unfortunately, scala does not support by-need evaluation of method parameters.

Scalaz formalises the three evaluation strategies with an ADT

  sealed abstract class Name[A] {
    def value: A
  object Name {
    def apply[A](a: =>A) = new Name[A] { def value = a }
  sealed abstract class Need[A] extends Name[A]
  object Need {
    def apply[A](a: =>A): Need[A] = new Need[A] {
      private lazy val value0: A = a
      def value = value0
  final case class Value[A](value: A) extends Need[A]

The weakest form of evaluation is Name, giving no computational guarantees. Next is Need, guaranteeing at most once evaluation, whereas Value is pre-computed and therefore exactly once evaluation.

If we wanted to be super-pedantic we could go back to all the typeclasses and make their methods take Name, Need or Value parameters. Instead we can assume that normal parameters can always be wrapped in a Value, and by-name parameters can be wrapped with Name.

When we write pure programs, we are free to replace any Name with Need or Value, and vice versa, with no change to the correctness of the program. This is the essence of referential transparency: the ability to replace a computation by its value, or a value by its computation.

In functional programming we almost always want Value or Need (also known as strict and lazy): there is little value in Name. Because there is no language level support for lazy method parameters, methods typically ask for a by-name parameter and then convert it into a Need internally, getting a boost to performance.

Name provides instances of the following typeclasses

  • Monad
  • Comonad
  • Distributive
  • Traverse1
  • Align
  • Zip / Unzip / Cozip

6.3 Memoisation

Scalaz has the capability to memoise functions, formalised by Memo, which doesn’t make any guarantees about evaluation because of the diversity of implementations:

  sealed abstract class Memo[K, V] {
    def apply(z: K => V): K => V
  object Memo {
    def memo[K, V](f: (K => V) => K => V): Memo[K, V]
    def nilMemo[K, V]: Memo[K, V] = memo[K, V](identity)
    def arrayMemo[V >: Null : ClassTag](n: Int): Memo[Int, V] = ...
    def doubleArrayMemo(n: Int, sentinel: Double = 0.0): Memo[Int, Double] = ...
    def immutableHashMapMemo[K, V]: Memo[K, V] = ...
    def immutableTreeMapMemo[K: scala.Ordering, V]: Memo[K, V] = ...

memo allows us to create custom implementations of Memo, nilMemo doesn’t memoise, evaluating the function normally. The remaining implementations intercept calls to the function and cache results backed by stdlib collection implementations.

To use Memo we simply wrap a function with a Memo implementation and then call the memoised function:

  scala> def foo(n: Int): String = {
           if (n > 10) "wibble" else "wobble"
  scala> val mem = Memo.arrayMemo[String](100)
         val mfoo = mem(foo)
  scala> mfoo(1)
  running // evaluated
  res: String = wobble
  scala> mfoo(1)
  res: String = wobble // memoised

If the function takes more than one parameter, we must tupled the method, with the memoised version taking a tuple.

  scala> def bar(n: Int, m: Int): String = "hello"
         val mem = Memo.immutableHashMapMemo[(Int, Int), String]
         val mbar = mem((bar _).tupled)
  scala> mbar((1, 2))
  res: String = "hello"

Memo is typically treated as a special construct and the usual rule about purity is relaxed for implementations. To be pure only requires that our implementations of Memo are referential transparent in the evaluation of K => V. We may use mutable data and perform I/O in the implementation of Memo, e.g. with an LRU or distributed cache, without having to declare an effect in the type signature. Other functional programming languages have automatic memoisation managed by their runtime environment and Memo is our way of extending the JVM to have similar support, unfortunately only on an opt-in basis.

6.4 Tagging

In the section introducing Monoid we built a Monoid[TradeTemplate] and realised that scalaz does not do what we wanted with Monoid[Option[A]]. This is not an oversight of scalaz: often we find that a data type can implement a fundamental typeclass in multiple valid ways and that the default implementation doesn’t do what we want, or simply isn’t defined.

Basic examples are Monoid[Boolean] (conjunction && vs disjunction ||) and Monoid[Int] (multiplication vs addition).

To implement Monoid[TradeTemplate] we found ourselves either breaking typeclass coherency, or using a different typeclass.

scalaz.Tag is designed to address the multiple typeclass implementation problem without breaking typeclass coherency.

The definition is quite contorted, but the syntax to use it is very clean. This is how we trick the compiler into allowing us to define an infix type A @@ T that is erased to A at runtime:

  type @@[A, T] = Tag.k.@@[A, T]
  object Tag {
    @inline val k: TagKind = IdTagKind
    @inline def apply[A, T](a: A): A @@ T = k(a)
    final class TagOf[T] private[Tag]() { ... }
    def of[T]: TagOf[T] = new TagOf[T]
  sealed abstract class TagKind {
    type @@[A, T]
    def apply[A, T](a: A): A @@ T
  private[scalaz] object IdTagKind extends TagKind {
    type @@[A, T] = A
    @inline override def apply[A, T](a: A): A = a

Some useful tags are provided in the Tags object

  object Tags {
    sealed trait First
    val First = Tag.of[First]
    sealed trait Last
    val Last = Tag.of[Last]
    sealed trait Multiplication
    val Multiplication = Tag.of[Multiplication]
    sealed trait Disjunction
    val Disjunction = Tag.of[Disjunction]
    sealed trait Conjunction
    val Conjunction = Tag.of[Conjunction]

First / Last are used to select Monoid instances that pick the first or last non-zero operand. Multiplication is for numeric multiplication instead of addition. Disjunction / Conjunction are to select && or ||, respectively.

In our TradeTemplate, instead of using Option[Currency] we can use Option[Currency] @@ Tags.Last. Indeed this is so common that we can use the built-in alias, LastOption

  type LastOption[A] = Option[A] @@ Tags.Last

letting us write a much cleaner Monoid[TradeTemplate]

  final case class TradeTemplate(
    payments: List[java.time.LocalDate],
    ccy: LastOption[Currency],
    otc: LastOption[Boolean]
  object TradeTemplate {
    implicit val monoid: Monoid[TradeTemplate] = Monoid.instance(
      (a, b) =>
        TradeTemplate(a.payments |+| b.payments,
                      a.ccy |+| b.ccy,
                      a.otc |+| b.otc),
        TradeTemplate(Nil, Tag(None), Tag(None))

To create a raw value of type LastOption, we apply Tag to an Option. Here we are calling Tag(None).

In the chapter on typeclass derivation, we’ll go one step further and automatically derive the monoid.

It is tempting to use Tag to markup data types for some form of validation (e.g. String @@ PersonName), but this should be avoided because there are no checks on the content of the runtime value. Tag should only be used for typeclass selection purposes, and we will dedicate a chapter on type refinement which is how we can mark up types with arbitrary value-level constraints.

6.5 Natural Transformations

A function from one type to another is written as A => B in scala, which is syntax sugar for a Function1[A, B]. Scalaz provides similar syntax sugar F ~> G for functions over type constructors F[_] to G[_].

These F ~> G are called natural transformations and are universally quantified because they don’t care about the contents of F[_].

  type ~>[-F[_], +G[_]] = NaturalTransformation[F, G]
  trait NaturalTransformation[-F[_], +G[_]] {
    def apply[A](fa: F[A]): G[A]
    def compose[E[_]](f: E ~> F): E ~> G = ...
    def andThen[H[_]](f: G ~> H): F ~> H = ...

An example of a natural transformation is a function that converts an IList into a List

  scala> val convert = new (IList ~> List) {
           def apply[A](fa: IList[A]): List[A] = fa.toList
  scala> convert(IList(1, 2, 3))
  res: List[Int] = List(1, 2, 3)

Or, more concisely, making use of kind-projector’s syntax sugar:

  scala> val convert = λ[IList ~> List](_.toList)
  scala> val convert = Lambda[IList ~> List](_.toList)

However, in day-to-day development, it is far more likely that we will use a natural transformation to map between algebras. For example, in drone-dynamic-agents we may want to implement our Google Container Engine Machines algebra with an off-the-shelf algebra, BigMachines. Instead of changing all our business logic and tests to use this new BigMachines interface, we may be able to write a transformation from Machines ~> BigMachines. We will return to this idea in the chapter on Advanced Monads.

There is also a natural transformation for type constructors with two type holes:

  type ~~>[-F[_,_], +G[_,_]] = BiNaturalTransformation[F, G]
  trait BiNaturalTransformation[-F[_, _], +G[_, _]] {
    def apply[A, B](f: F[A, B]): G[A, B]
    def compose[E[_, _]](f: E ~~> F) = ...

6.6 Isomorphism

Sometimes we have two types that are really the same thing, causing compatibility problems because the compiler doesn’t know what we know. This typically happens when we use third party code that is the same as something we already have.

This is when Isomorphism can help us out. An isomorphism defines a formal “is equivalent to” relationship between two types. There are three variants, to account for types of different shapes:

  object Isomorphism {
    trait Iso[Arr[_, _], A, B] {
      def to: Arr[A, B]
      def from: Arr[B, A]
    type IsoSet[A, B] = Iso[Function1, A, B]
    type <=>[A, B] = IsoSet[A, B]
    object IsoSet {
      def apply[A, B](to: A => B, from: B => A): A <=> B = ...
    trait Iso2[Arr[_[_], _[_]], F[_], G[_]] {
      def to: Arr[F, G]
      def from: Arr[G, F]
    type IsoFunctor[F[_], G[_]] = Iso2[NaturalTransformation, F, G]
    type <~>[F[_], G[_]] = IsoFunctor[F, G]
    object IsoFunctor {
      def apply[F[_], G[_]](to: F ~> G, from: G ~> F): F <~> G = ...
    trait Iso3[Arr[_[_, _], _[_, _]], F[_, _], G[_, _]] {
      def to: Arr[F, G]
      def from: Arr[G, F]
    type IsoBifunctor[F[_, _], G[_, _]] = Iso3[~~>, F, G]
    type <~~>[F[_, _], G[_, _]] = IsoBifunctor[F, G]

The type aliases IsoSet, IsoFunctor and IsoBifunctor cover the common cases: a regular function, natural transformation and binatural. Convenience functions allow us to generate instances from existing functions or natural transformations. However, it is often easier to use one of the abstract Template classes to define an isomorphism. For example:

  val listIListIso: List <~> IList =
    new IsoFunctorTemplate[List, IList] {
      def to[A](fa: List[A]) = fromList(fa)
      def from[A](fa: IList[A]) = fa.toList

If we introduce an isomorphism, we can generate many of the standard typeclasses. For example

  trait IsomorphismSemigroup[F, G] extends Semigroup[F] {
    implicit def G: Semigroup[G]
    def iso: F <=> G
    def append(f1: F, f2: => F): F = iso.from(G.append(,

allows us to derive a Semigroup[F] for a type F if we have an F <=> G and a Semigroup[G]. Almost all the typeclasses in the hierarchy provide an isomorphic variant. If we find ourselves copying and pasting a typeclass implementation, it is worth considering if Isomorphism is the better solution.

6.7 Containers

6.7.1 Maybe

We have already encountered scalaz’s improvement over scala.Option, called Maybe. It is an improvement because it does not have any unsafe methods like Option.get, which can throw an exception, and is invariant.

It is typically used to represent when a thing may be present or not without giving any extra context as to why it may be missing.

  sealed abstract class Maybe[A] { ... }
  object Maybe {
    final case class Empty[A]()    extends Maybe[A]
    final case class Just[A](a: A) extends Maybe[A]
    def empty[A]: Maybe[A] = Empty()
    def just[A](a: A): Maybe[A] = Just(a)
    def fromOption[A](oa: Option[A]): Maybe[A] = ...
    def fromNullable[A](a: A): Maybe[A] = if (null == a) empty else just(a)

The .empty and .just companion methods are preferred to creating raw Empty or Just instances because they return a Maybe, helping with type inference. This pattern is often referred to as returning a sum type, which is when we have multiple implementations of a sealed trait but never use a specific subtype in a method signature.

A convenient implicit class allows us to call .just on any value and receive a Maybe

  implicit class MaybeOps[A](self: A) {
    def just: Maybe[A] = Maybe.just(self)

Maybe has a typeclass instance for all the things

  • Align
  • Traverse
  • MonadPlus / IsEmpty
  • Cobind
  • Cozip / Zip / Unzip
  • Optional

and delegate instances depending on A

  • Monoid / Band / SemiLattice
  • Equal / Order / Show

In addition to the above, Maybe has some niche functionality that is not supported by a polymorphic typeclass.

  sealed abstract class Maybe[A] {
    def cata[B](f: A => B, b: =>B): B = this match {
      case Just(a) => f(a)
      case Empty() => b
    def |(a: =>A): A = cata(identity, a)
    def toLeft[B](b: =>B): A \/ B = cata(\/.left, \/-(b))
    def toRight[B](b: =>B): B \/ A = cata(\/.right, -\/(b))
    def <\/[B](b: =>B): A \/ B = toLeft(b)
    def \/>[B](b: =>B): B \/ A = toRight(b)
    def orZero(implicit A: Monoid[A]): A = getOrElse(
    def unary_~(implicit A: Monoid[A]): A = orZero
    def orEmpty[F[_]: Applicative: PlusEmpty]: F[A] =
      cata(Applicative[F].point(_), PlusEmpty[F].empty)

.cata is a terser alternative to .map(f).getOrElse(b) and has the simpler form | if the map is identity (i.e. just .getOrElse).

.toLeft and .toRight, and their symbolic aliases, create a disjunction (explained in the next section) by taking a fallback for the Empty case.

.orZero (having ~foo syntax) takes a Monoid to define the default value.

.orEmpty uses an ApplicativePlus to create a single element or empty container, not forgetting that we already get support for stdlib collections from the Foldable instance’s .to method.

  scala> ~1.just
  res: Int = 1
  scala> Maybe.empty[Int].orZero
  res: Int = 0
  scala> Maybe.empty[Int].orEmpty[IList]
  res: IList[Int] = []
  scala> 1.just.orEmpty[IList]
  res: IList[Int] = [1]
  scala>[List] // from Foldable
  res: List[Int] = List(1)

6.7.2 Either

Scalaz’s improvement over scala.Either is symbolic, but it is common to speak about it as either or Disjunction

  sealed abstract class \/[+A, +B] { ... }
  final case class -\/[+A](a: A) extends (A \/ Nothing)
  final case class \/-[+B](b: B) extends (Nothing \/ B)
  type Disjunction[+A, +B] = \/[A, B]
  type DLeft[+A] = -\/[A]
  type DRight[+B] = \/-[B]
  object \/ {
    def left [A, B]: A => A \/ B = -\/(_)
    def right[A, B]: B => A \/ B = \/-(_)
    def fromEither[A, B](e: Either[A, B]): A \/ B = ...

with corresponding syntax

  implicit class EitherOps[A](val self: A) {
    final def left [B]: (A \/ B) = -\/(self)
    final def right[B]: (B \/ A) = \/-(self)

allowing for easy construction of values. Note that the extension method takes the type of the other side. So if we wish to create a String \/ Int and we have an Int, we must pass String when calling .right

  scala> 1.right[String]
  res: String \/ Int = \/-(1)
  scala> "hello".left[Int]
  res: String \/ Int = -\/(hello)

The symbolic nature of \/ makes it read well in type signatures when shown infix. Note that symbolic types in Scala associate from the left and nested \/ must have parentheses, e.g. (A \/ (B \/ (C \/ D)).

\/ has right-biased (i.e. flatMap applies to \/-) typeclass instances for:

  • Monad / MonadError
  • Traverse / Bitraverse
  • Plus
  • Optional
  • Cozip

and depending on the contents

  • Equal / Order
  • Semigroup / Monoid / Band

In addition, there are custom methods

  sealed abstract class \/[+A, +B] { self =>
    def fold[X](l: A => X, r: B => X): X = self match {
      case -\/(a) => l(a)
      case \/-(b) => r(b)
    def swap: (B \/ A) = self match {
      case -\/(a) => \/-(a)
      case \/-(b) => -\/(b)
    def unary_~ : (B \/ A) = swap
    def |[BB >: B](x: =>BB): BB = getOrElse(x) // Optional[_]
    def |||[C, BB >: B](x: =>C \/ BB): C \/ BB = orElse(x) // Optional[_]
    def +++[AA >: A: Semigroup, BB >: B: Semigroup](x: =>AA \/ BB): AA \/ BB = ...
    def toEither: Either[A, B] = ...
    final class SwitchingDisjunction[X](right: =>X) {
      def <<?:(left: =>X): X = ...
    def :?>>[X](right: =>X) = new SwitchingDisjunction[X](right)

.fold is similar to Maybe.cata and requires that both the left and right sides are mapped to the same type.

.swap (and the ~foo syntax) swaps a left into a right and a right into a left.

The | alias to getOrElse appears similarly to Maybe. We also get ||| as an alias to orElse.

+++ is for combining disjunctions with lefts taking preference over right:

  • right(v1) +++ right(v2) gives right(v1 |+| v2)
  • right(v1) +++ left (v2) gives left (v2)
  • left (v1) +++ right(v2) gives left (v1)
  • left (v1) +++ left (v2) gives left (v1 |+| v2)

.toEither is provided for backwards compatibility with the Scala stdlib.

The combination of :?>> and <<?: allow for a convenient syntax to ignore the contents of an \/, but pick a default based on its type

  scala> 1 <<?: foo :?>> 2
  res: Int = 2 // foo is a \/-
  scala> 1 <<?: ~foo :?>> 2
  res: Int = 1

Scalaz also comes with Either3, for storing one of three values

  sealed abstract class Either3[+A, +B, +C]
  final case class Left3[+A, +B, +C](a: A)   extends Either3[A, B, C]
  final case class Middle3[+A, +B, +C](b: B) extends Either3[A, B, C]
  final case class Right3[+A, +B, +C](c: C)  extends Either3[A, B, C]

However, it is common to use nested disjunctions A \/ (B \/ C), making Either3 redundant.

6.7.3 Validation

At first sight, Validation (aliased with \?/, happy Elvis) appears to be a clone of Disjunction:

  sealed abstract class Validation[+E, +A] { ... }
  final case class Success[A](a: A) extends Validation[Nothing, A]
  final case class Failure[E](e: E) extends Validation[E, Nothing]
  type ValidationNel[E, +X] = Validation[NonEmptyList[E], X]
  object Validation {
    type \?/[+E, +A] = Validation[E, A]
    def success[E, A]: A => Validation[E, A] = Success(_)
    def failure[E, A]: E => Validation[E, A] = Failure(_)
    def failureNel[E, A](e: E): ValidationNel[E, A] = Failure(NonEmptyList(e))
    def lift[E, A](a: A)(f: A => Boolean, fail: E): Validation[E, A] = ...
    def liftNel[E, A](a: A)(f: A => Boolean, fail: E): ValidationNel[E, A] = ...
    def fromEither[E, A](e: Either[E, A]): Validation[E, A] = ...

With convenient syntax

  implicit class ValidationOps[A](self: A) {
    def success[X]: Validation[X, A] = Validation.success[X, A](self)
    def successNel[X]: ValidationNel[X, A] = success
    def failure[X]: Validation[A, X] = Validation.failure[A, X](self)
    def failureNel[X]: ValidationNel[A, X] = Validation.failureNel[A, X](self)

However, the data structure itself is not the complete story. Validation intentionally does not have an instance of any Monad, restricting itself to success-biased versions of:

  • Applicative
  • Traverse / Bitraverse
  • Cozip
  • Plus
  • Optional

and depending on the contents

  • Equal / Order
  • Show
  • Semigroup / Monoid

The big advantage of restricting to Applicative is that Validation is explicitly for situations where we wish to report all failures, whereas Disjunction is used to stop at the first failure. To accommodate failure accumulation, a popular form of Validation is ValidationNel, having a NonEmptyList[E] in the failure position.

Consider performing input validation of data provided by a user using Disjunction and flatMap:

  scala> :paste
         final case class Credentials(user: Username, name: Fullname)
         final case class Username(value: String) extends AnyVal
         final case class Fullname(value: String) extends AnyVal
         def username(in: String): String \/ Username =
           if (in.isEmpty) "empty username".left
           else if (in.contains(" ")) "username contains spaces".left
           else Username(in).right
         def realname(in: String): String \/ Fullname =
           if (in.isEmpty) "empty real name".left
           else Fullname(in).right
  scala> for {
           u <- username("sam halliday")
           r <- realname("")
         } yield Credentials(u, r)
  res = -\/(username contains spaces)

If we use |@| syntax

  scala> (username("sam halliday") |@| realname("")) (Credentials.apply)
  res = -\/(username contains spaces)

we still get back the first failure. This is because Disjunction is a Monad, its .applyX methods must be consistent with .flatMap and not assume that any operations can be performed out of order. Compare to:

  scala> :paste
         def username(in: String): ValidationNel[String, Username] =
           if (in.isEmpty) "empty username".failureNel
           else if (in.contains(" ")) "username contains spaces".failureNel
           else Username(in).success
         def realname(in: String): ValidationNel[String, Fullname] =
           if (in.isEmpty) "empty real name".failureNel
           else Fullname(in).success
  scala> (username("sam halliday") |@| realname("")) (Credentials.apply)
  res = Failure(NonEmpty[username contains spaces,empty real name])

This time, we get back all the failures!

Validation has many of the same methods as Disjunction, such as .fold, .swap and +++, plus some extra:

  sealed abstract class Validation[+E, +A] {
    def append[F >: E: Semigroup, B >: A: Semigroup](x: F \?/ B]): F \?/ B = ...
    def disjunction: (E \/ A) = ...

.append (aliased by +|+) has the same type signature as +++ but prefers the success case

  • failure(v1) +|+ failure(v2) gives failure(v1 |+| v2)
  • failure(v1) +|+ success(v2) gives success(v2)
  • success(v1) +|+ failure(v2) gives success(v1)
  • success(v1) +|+ success(v2) gives success(v1 |+| v2)

.disjunction converts a Validated[A, B] into an A \/ B. Disjunction has the mirror .validation and .validationNel to convert into Validation, allowing for easy conversion between sequential and parallel failure accumulation.

\/ and Validation are the more performant FP equivalent of a checked exception for input validation, avoiding both a stacktrace and requiring the caller to deal with the failure resulting in more robust systems.

6.7.4 These

We encountered These, a data encoding of inclusive logical OR, when we learnt about Align. Let’s take a closer look:

  sealed abstract class \&/[+A, +B] { ... }
  object \&/ {
    type These[A, B] = A \&/ B
    final case class This[A](aa: A) extends (A \&/ Nothing)
    final case class That[B](bb: B) extends (Nothing \&/ B)
    final case class Both[A, B](aa: A, bb: B) extends (A \&/ B)
    def apply[A, B](a: A, b: B): These[A, B] = Both(a, b)

with convenient construction syntax

  implicit class TheseOps[A](self: A) {
    final def wrapThis[B]: A \&/ B = \&/.This(self)
    final def `this`[B]: A \&/ B = wrapThis
    final def wrapThat[B]: B \&/ A = \&/.That(self)
    final def that[B]: B \&/ A = wrapThat
  implicit class ThesePairOps[A, B](self: (A, B)) {
    final def both: A \&/ B = \&/.Both(self._1, self._2)

Annoyingly, this is a keyword in Scala and must be called with back-ticks, or as .wrapThis.

These has typeclass instances for

  • Monad
  • Bitraverse
  • Traverse
  • Cobind

and depending on contents

  • Semigroup / Monoid / Band
  • Equal / Order
  • Show

These (\&/) has many of the methods we have come to expect of Disjunction (\/) and Validation (\?/)

  sealed abstract class \&/[+A, +B] {
    def fold[X](s: A => X, t: B => X, q: (A, B) => X): X = ...
    def swap: (B \&/ A) = ...
    def unary_~ : (B \&/ A) = swap
    def append[X >: A: Semigroup, Y >: B: Semigroup](o: =>(X \&/ Y)): X \&/ Y = ...
    def &&&[X >: A: Semigroup, C](t: X \&/ C): X \&/ (B, C) = ...

.append has 9 possible arrangements and data is never thrown away because cases of This and That can always be converted into a Both.

.flatMap is right-biased (Both and That), taking a Semigroup of the left content (This) to combine rather than break early. &&& is a convenient way of binding over two of these, creating a tuple on the right and dropping data if it is not present in each of these.

Although it is tempting to use \&/ in return types, overuse is an anti-pattern. The main reason to use \&/ is to combine or split potentially infinite streams of data in finite memory. Convenient functions exist on the companion to deal with EphemeralStream (aliased here to fit in a single line) or anything with a MonadPlus

  type EStream[A] = EphemeralStream[A]
  object \&/ {
    def concatThisStream[A, B](x: EStream[A \&/ B]): EStream[A] = ...
    def concatThis[F[_]: MonadPlus, A, B](x: F[A \&/ B]): F[A] = ...
    def concatThatStream[A, B](x: EStream[A \&/ B]): EStream[B] = ...
    def concatThat[F[_]: MonadPlus, A, B](x: F[A \&/ B]): F[B] = ...
    def unalignStream[A, B](x: EStream[A \&/ B]): (EStream[A], EStream[B]) = ...
    def unalign[F[_]: MonadPlus, A, B](x: F[A \&/ B]): (F[A], F[B]) = ...
    def merge[A: Semigroup](t: A \&/ A): A = ...

6.7.5 Higher Kinded Either

The Coproduct data type (not to be confused with the more general concept of a coproduct in an ADT, or shapeless.Coproduct) wraps Disjunction for type constructors:

  final case class Coproduct[F[_], G[_], A](run: F[A] \/ G[A]) { ... }
  object Coproduct {
    def leftc[F[_], G[_], A](x: F[A]): Coproduct[F, G, A] = Coproduct(-\/(x))
    def rightc[F[_], G[_], A](x: G[A]): Coproduct[F, G, A] = Coproduct(\/-(x))

Typeclass instances simply delegate to those of the F[_] and G[_].

The most popular use case for Coproduct is when we want to create an anonymous coproduct for a GADT.

6.7.6 Not So Eager

Built-in Scala tuples, and basic data types like Maybe and Disjunction are eagerly-evaluated value types.

For convenience, by-name alternatives to Name are provided, having the expected typeclass instances:

  sealed abstract class LazyTuple2[A, B] {
    def _1: A
    def _2: B
  sealed abstract class LazyTuple4[A, B, C, D] {
    def _1: A
    def _2: B
    def _3: C
    def _4: D
  sealed abstract class LazyOption[+A] { ... }
  private final case class LazySome[A](a: () => A) extends LazyOption[A]
  private case object LazyNone extends LazyOption[Nothing]
  sealed abstract class LazyEither[+A, +B] { ... }
  private case class LazyLeft[A, B](a: () => A) extends LazyEither[A, B]
  private case class LazyRight[A, B](b: () => B) extends LazyEither[A, B]

The astute reader will note that Lazy* is a misnomer, and these data types should perhaps be: ByNameTupleX, ByNameOption and ByNameEither.

6.7.7 Const

Const, for constant, is a wrapper for a value of type A, along with a spare type parameter B.

  final case class Const[A, B](getConst: A)

Const provides an instance of Applicative[Const[A, ?]] if there is a Monoid[A] available:

  implicit def applicative[A: Monoid]: Applicative[Const[A, ?]] =
    new Applicative[Const[A, ?]] {
      def point[B](b: =>B): Const[A, B] =
      def ap[B, C](fa: =>Const[A, B])(fbc: =>Const[A, B => C]): Const[A, C] =
        Const(fbc.getConst |+| fa.getConst)

The most important thing about this Applicative is that it ignores the B parameters, continuing on without failing and only combining the constant values that it encounters.

Going back to our example application drone-dynamic-agents, we should first refactor our logic.scala file to use Applicative instead of Monad. We wrote logic.scala before we learnt about Applicative and now we know better:

  final class DynAgents[F[_]: Applicative](implicit d: Drone[F], m: Machines[F]) {
    def act(world: WorldView): F[WorldView] = world match {
      case NeedsAgent(node) =>
        m.start(node) >| world.copy(pending = Map(node -> world.time))
      case Stale(nodes) =>
        nodes.traverse { node =>
          m.stop(node) >| node
        }.map { stopped =>
          val updates = stopped.strengthR(world.time).toList.toMap
          world.copy(pending = world.pending ++ updates)
      case _ => world.pure[F]

Since our business logic only requires an Applicative, we can write mock implementations with F[a] as Const[String, a]. In each case, we return the name of the function that is called:

  object ConstImpl {
    type F[a] = Const[String, a]
    implicit val drone: Drone[F] = new Drone[F] {
      def getBacklog: F[Int] = Const("backlog")
      def getAgents: F[Int]  = Const("agents")
    implicit val machines: Machines[F] = new Machines[F] {
      def getAlive: F[Map[MachineNode, Instant]]   = Const("alive")
      def getManaged: F[NonEmptyList[MachineNode]] = Const("managed")
      def getTime: F[Instant]                      = Const("time")
      def start(node: MachineNode): F[Unit]        = Const("start")
      def stop(node: MachineNode): F[Unit]         = Const("stop")
    val program = new DynAgents[F]

With this interpretation of our program, we can assert on the methods that are called:

  it should "call the expected methods" in {
    import ConstImpl._
    val alive    = Map(node1 -> time1, node2 -> time1)
    val world    = WorldView(1, 1, managed, alive, Map.empty, time4)
    program.act(world).getConst shouldBe "stopstop"

Alternatively, we could have counted total method calls by using Const[Int, ?] or an IMap[String, Int].

With this test, we’ve gone beyond traditional Mock testing with a Const test that asserts on what is called without having to provide implementations. This is useful if our specification demands that we make certain calls for certain input, e.g. for accounting purposes. Furthermore, we’ve achieved this with compiletime safety.

Let’s take this line of thinking a little further and say we want to monitor (in production) the nodes that we are stopping in act. We can create implementations of Drone and Machines with Const, calling it from our wrapped version of act

  final class Monitored[U[_]: Functor](program: DynAgents[U]) {
    type F[a] = Const[Set[MachineNode], a]
    implicit val drone: Drone[F] = new Drone[F] {
      def getBacklog: F[Int] = Const(Set.empty)
      def getAgents: F[Int]  = Const(Set.empty)
    implicit val machines: Machines[F] = new Machines[F] {
      def getAlive: F[Map[MachineNode, Instant]]   = Const(Set.empty)
      def getManaged: F[NonEmptyList[MachineNode]] = Const(Set.empty)
      def getTime: F[Instant]                      = Const(Set.empty)
      def start(node: MachineNode): F[Unit]        = Const(Set.empty)
      def stop(node: MachineNode): F[Unit]         = Const(Set(node))
    val monitor = new DynAgents[F]
    def act(world: WorldView): U[(WorldView, Set[MachineNode])] = {
      val stopped = monitor.act(world).getConst

We can do this because monitor is pure and running it produces no side effects.

This runs the program with ConstImpl, extracting all the calls to Machines.stop, then returning it alongside the WorldView. We can unit test this:

  it should "monitor stopped nodes" in {
    val underlying = new Mutable(needsAgents).program
    val alive = Map(node1 -> time1, node2 -> time1)
    val world = WorldView(1, 1, managed, alive, Map.empty, time4)
    val expected = world.copy(pending = Map(node1 -> time4, node2 -> time4))
    val monitored = new Monitored(underlying)
    monitored.act(world) shouldBe (expected -> Set(node1, node2))

We have used Const to do something that looks like Aspect Oriented Programming, once popular in Java. We built on top of our business logic to support a monitoring concern, without having to complicate the business logic.

It gets even better. We can run ConstImpl in production to gather what we want to stop, and then provide an optimised implementation of act that can make use of implementation-specific batched calls.

The silent hero of this story is Applicative. Const lets us show off what is possible. If we need to change our program to require a Monad, we can no longer use Const and must write full mocks to be able to assert on what is called under certain inputs. The Rule of Least Power demands that we use Applicative instead of Monad wherever we can.

6.8 Collections

Unlike the stdlib Collections API, the scalaz approach describes collection behaviours in the typeclass hierarchy, e.g. Foldable, Traverse, Monoid. What remains to be studied are the implementations in terms of data structures, which have different performance characteristics and niche methods.

Because all the collection data types provide more or less the same list of typeclass instances, we shall avoid repeating the list, which is often some variation of:

  • Monoid
  • Traverse / Foldable
  • MonadPlus / IsEmpty
  • Cobind / Comonad
  • Zip / Unzip
  • Align
  • Equal / Order
  • Show

Data structures that are provably non-empty are able to provide

  • Traverse1 / Foldable1

and provide Semigroup instead of Monoid, Plus instead of IsEmpty.

6.8.1 Lists

We have used IList[A] and NonEmptyList[A] so many times by now that they should be familiar. They codify a classic linked list data structure:

  sealed abstract class IList[A] {
    def ::(a: A): IList[A] = ...
    def :::(as: IList[A]): IList[A] = ...
    def toList: List[A] = ...
    def toNel: Option[NonEmptyList[A]] = ...
  final case class INil[A]() extends IList[A]
  final case class ICons[A](head: A, tail: IList[A]) extends IList[A]
  final case class NonEmptyList[A](head: A, tail: IList[A]) {
    def <::(b: A): NonEmptyList[A] = nel(b, head :: tail)
    def <:::(bs: IList[A]): NonEmptyList[A] = ...

The main advantage of IList over stdlib List is that there are no unsafe methods, like .head which throws an exception on an empty list.

In addition, IList is a lot simpler, having no hierarchy and a much smaller bytecode footprint. Furthermore, the stdlib List has a terrifying implementation that uses var to workaround performance problems in the stdlib collection design:

  package scala.collection.immutable
  sealed abstract class List[+A]
    extends AbstractSeq[A]
    with LinearSeq[A]
    with GenericTraversableTemplate[A, List]
    with LinearSeqOptimized[A, List[A]] { ... }
  case object Nil extends List[Nothing] { ... }
  final case class ::[B](
    override val head: B,
    private[scala] var tl: List[B]
  ) extends List[B] { ... }

List creation requires careful, and slow, Thread synchronisation to ensure safe publishing. IList requires no such hacks and can therefore outperform List.

6.8.2 EphemeralStream

The stdlib Stream is a lazy version of List, but is riddled with memory leaks and unsafe methods. EphemeralStream does not keep references to computed values, helping to alleviate the memory retention problem, and removing unsafe methods in the same spirit as IList.

  sealed abstract class EphemeralStream[A] {
    def headOption: Option[A]
    def tailOption: Option[EphemeralStream[A]]
  // private implementations
  object EphemeralStream extends EphemeralStreamInstances {
    type EStream[A] = EphemeralStream[A]
    def emptyEphemeralStream[A]: EStream[A] = ...
    def cons[A](a: =>A, as: =>EStream[A]): EStream[A] = ...
    def unfold[A, B](start: =>B)(f: B => Option[(A, B)]): EStream[A] = ...
    def iterate[A](start: A)(f: A => A): EStream[A] = ...
    implicit class ConsWrap[A](e: =>EStream[A]) {
      def ##::(h: A): EStream[A] = cons(h, e)
    object ##:: {
      def unapply[A](xs: EStream[A]): Option[(A, EStream[A])] =
        if (xs.isEmpty) None
        else Some((xs.head(), xs.tail()))

.cons, .unfold and .iterate are mechanisms for creating streams, and the convenient syntax ##:: puts a new element at the head of a by-name EStream reference. .unfold is for creating a finite (but possibly infinite) stream by repeatedly applying a function f to get the next value and input for the following f. .iterate creates an infinite stream by repeating a function f on the previous element.

EStream may appear in pattern matches with the symbol ##::, matching the syntax for .cons.

Although EStream addresses the value memory retention problem, it is still possible to suffer from slow memory leaks if a live reference points to the head of an infinite stream. Problems of this nature, as well as the need to compose effectful streams, are why fs2 exists.

6.8.3 CorecursiveList

Corecursion is when we start from a base state and produce subsequent steps deterministically, like the EphemeralStream.unfold method that we just studied:

  def unfold[A, B](b: =>B)(f: B => Option[(A, B)]): EStream[A] = ...

Contrast to recursion, which breaks data into a base state and then terminates.

A CorecursiveList is a data encoding of EphemeralStream.unfold, offering an alternative to EStream that may perform better in some circumstances:

  sealed abstract class CorecursiveList[A] {
    type S
    def init: S
    def step: S => Maybe[(S, A)]
  object CorecursiveList {
    private final case class CorecursiveListImpl[S0, A](
      init: S0,
      step: S0 => Maybe[(S0, A)]
    ) extends CorecursiveList[A] { type S = S0 }
    def apply[S, A](init: S)(step: S => Maybe[(S, A)]): CorecursiveList[A] =
      CorecursiveListImpl(init, step)

Corecursion is useful when implementing Comonad.cojoin, like our Hood example. CorecursiveList is a good way to codify non-linear recurrence equations like those used in biology population models, drone control systems, macro economics, and investment banking models.

6.8.4 ImmutableArray

A simple wrapper around mutable stdlib Array, with primitive specialisations:

  sealed abstract class ImmutableArray[+A] {
    def ++[B >: A: ClassTag](o: ImmutableArray[B]): ImmutableArray[B]
  object ImmutableArray {
    final class StringArray(s: String) extends ImmutableArray[Char] { ... }
    sealed class ImmutableArray1[+A](as: Array[A]) extends ImmutableArray[A] { ... }
    final class ofRef[A <: AnyRef](as: Array[A]) extends ImmutableArray1[A](as)
    final class ofLong(as: Array[Long]) extends ImmutableArray1[Long](as)
    def fromArray[A](x: Array[A]): ImmutableArray[A] = ...
    def fromString(str: String): ImmutableArray[Char] = ...

Array is unrivalled in terms of read performance and heap size. However, there is zero structural sharing when creating new arrays, therefore arrays are typically used only when their contents are not expected to change, or as a way of safely wrapping raw data from a legacy system.

6.8.5 Dequeue

A Dequeue (pronounced like a “deck” of cards) is a linked list that allows items to be put onto or retrieved from the front (cons) or the back (snoc) in constant time. Removing an element from either end is constant time on average.

  sealed abstract class Dequeue[A] {
    def frontMaybe: Maybe[A]
    def backMaybe: Maybe[A]
    def ++(o: Dequeue[A]): Dequeue[A] = ...
    def +:(a: A): Dequeue[A] = cons(a)
    def :+(a: A): Dequeue[A] = snoc(a)
    def cons(a: A): Dequeue[A] = ...
    def snoc(a: A): Dequeue[A] = ...
    def uncons: Maybe[(A, Dequeue[A])] = ...
    def unsnoc: Maybe[(A, Dequeue[A])] = ...
  private final case class SingletonDequeue[A](single: A) extends Dequeue[A] { ... }
  private final case class FullDequeue[A](
    front: NonEmptyList[A],
    fsize: Int,
    back: NonEmptyList[A],
    backSize: Int) extends Dequeue[A] { ... }
  private final case object EmptyDequeue extends Dequeue[Nothing] { ... }
  object Dequeue {
    def empty[A]: Dequeue[A] = EmptyDequeue()
    def apply[A](as: A*): Dequeue[A] = ...
    def fromFoldable[F[_]: Foldable, A](fa: F[A]): Dequeue[A] = ...

The way it works is that there are two lists, one for the front data and another for the back. Consider an instance holding symbols a0, a1, a2, a3, a4, a5, a6

    NonEmptyList('a0, IList('a1, 'a2, 'a3)), 4,
    NonEmptyList('a6, IList('a5, 'a4)), 3)

which can be visualised as

Note that the list holding the back is in reverse order.

Reading the snoc (final element) is a simple lookup into back.head. Adding an element to the end of the Dequeue means adding a new element to the head of the back, and recreating the FullDequeue wrapper (which will increase backSize by one). Almost all of the original structure is shared. Compare to adding a new element to the end of an IList, which would involve recreating the entire structure.

The frontSize and backSize are used to re-balance the front and back so that they are always approximately the same size. Re-balancing means that some operations can be slower than others (e.g. when the entire structure must be rebuilt) but because it happens only occasionally, we can take the average of the cost and say that it is constant.

6.8.6 DList

Linked lists have poor performance characteristics when large lists are appended together. Consider the work that goes into evaluating the following:

  ((as ::: bs) ::: (cs ::: ds)) ::: (es ::: (fs ::: gs))

This creates six intermediate lists, traversing and rebuilding every list three times (except for gs which is shared between all stages).

The DList (for difference list) is a more efficient solution for this scenario. Instead of performing the calculations at each stage, it is represented as a function IList[A] => IList[A]

  final case class DList[A](f: IList[A] => IList[A]) {
    def toIList: IList[A] = f(IList.empty)
    def ++(as: DList[A]): DList[A] = DList(xs => f(as.f(xs)))
  object DList {
    def fromIList[A](as: IList[A]): DList[A] = DList(xs => as ::: xs)

The equivalent calculation is (the symbols created via DList.fromIList)

  (((a ++ b) ++ (c ++ d)) ++ (e ++ (f ++ g))).toIList

which breaks the work into right-associative (i.e. fast) appends

  (as ::: (bs ::: (cs ::: (ds ::: (es ::: (fs ::: gs))))))

utilising the fast constructor on IList.

As always, there is no free lunch. There is a memory allocation overhead that can slow down code that naturally results in right-associative appends. The largest speedup is when IList operations are left-associative, e.g.

  ((((((as ::: bs) ::: cs) ::: ds) ::: es) ::: fs) ::: gs)

Difference lists suffer from bad marketing. If they were called a ListBuilderFactory they’d probably be in the standard library.

6.8.7 ISet

Tree structures are excellent for storing ordered data, with every binary node holding elements that are less than in one branch, and greater than in the other. However, naive implementations of a tree structure can become unbalanced depending on the insertion order. It is possible to maintain a perfectly balanced tree, but it is incredibly inefficient as every insertion effectively rebuilds the entire tree.

ISet is an implementation of a tree of bounded balance, meaning that it is approximately balanced, using the size of each branch to balance a node.

  sealed abstract class ISet[A] {
    val size: Int = this match {
      case Tip()        => 0
      case Bin(_, l, r) => 1 + l.size + r.size
  object ISet {
    private final case class Tip[A]() extends ISet[A]
    private final case class Bin[A](a: A, l: ISet[A], r: ISet[A]) extends ISet[A]
    def empty[A]: ISet[A] = Tip()
    def singleton[A](x: A): ISet[A] = Bin(x, Tip(), Tip())
    def fromFoldable[F[_]: Foldable, A: Order](xs: F[A]): ISet[A] =
      xs.foldLeft(empty[A])((a, b) => a insert b)

ISet requires A to have an Order. The Order[A] instance must remain the same between calls or internal assumptions will be invalid, leading to data corruption: i.e. we are assuming typeclass coherence such that Order[A] is unique for any A.

The ISet ADT unfortunately permits invalid trees. We strive to write ADTs that fully describe what is and isn’t valid through type restrictions, but sometimes there are situations where it can only be achieved by the inspired touch of an immortal. Instead, Tip / Bin are private, to stop users from accidentally constructing invalid trees. .insert is the only way to build an ISet, therefore defining what constitutes a valid tree.

  sealed abstract class ISet[A] {
    def contains(x: A)(implicit o: Order[A]): Boolean = ...
    def union(other: ISet[A])(implicit o: Order[A]): ISet[A] = ...
    def delete(x: A)(implicit o: Order[A]): ISet[A] = ...
    def insert(x: A)(implicit o: Order[A]): ISet[A] = this match {
      case Tip() => ISet.singleton(x)
      case self @ Bin(y, l, r) => o.order(x, y) match {
        case LT => balanceL(y, l.insert(x), r)
        case GT => balanceR(y, l, r.insert(x))
        case EQ => self

The internal methods .balanceL and .balanceR are mirrors of each other, so we only study .balanceL, which is called when the value we are inserting is less than the current node. It is also called by the .delete method.

  def balanceL[A](y: A, left: ISet[A], right: ISet[A]): ISet[A] = (left, right) match {

Balancing requires us to classify the scenarios that can occur. We will go through each possible scenario, visualising the (y, left, right) on the left side of the page, with the balanced structure on the right, also known as the rotated tree.

  • filled circles visualise a Tip
  • three columns visualise the left | value | right fields of Bin
  • diamonds visualise any ISet

The first scenario is the trivial case, which is when both the left and right are Tip. In fact we will never encounter this scenario from .insert, but we hit it in .delete

  case (Tip(), Tip()) => singleton(y)

The second case is when left is a Bin containing only Tip, we don’t need to balance anything, we just create the obvious connection:

  case (Bin(lx, Tip(), Tip()), Tip()) => Bin(y, left, Tip())

The third case is when it starts to get interesting: left is a Bin containing a Bin in its right

  case (Bin(lx, Tip(), Bin(lrx, _, _)), Tip()) =>
    Bin(lrx, singleton(lx), singleton(y))

But what happened to the two diamonds sitting below lrx? Didn’t we just lose information? No, we didn’t lose information, because we can reason (based on size balancing) that they are always Tip! There is no rule in any of the following scenarios (or in .balanceR) that can produce a tree of the shape where the diamonds are Bin.

The fourth case is the opposite of the third case.

  case (Bin(lx, ll, Tip()), Tip()) => Bin(lx, ll, singleton(y))

The fifth case is when we have full trees on both sides of the left and we must use their relative sizes to decide on how to re-balance.

  case (Bin(lx, ll, lr), Tip()) if (2*ll.size > lr.size) =>
    Bin(lx, ll, Bin(y, lr, Tip()))
  case (Bin(lx, ll, Bin(lrx, lrl, lrr)), Tip()) =>
    Bin(lrx, Bin(lx, ll, lrl), Bin(y, lrr, Tip()))

For the first branch, 2*ll.size > lr.size

and for the second branch 2*ll.size <= lr.size

The sixth scenario introduces a tree on the right. When the left is empty we create the obvious connection. This scenario never arises from .insert because the left is always non-empty:

  case (Tip(), r) => Bin(y, Tip(), r)

The final scenario is when we have non-empty trees on both sides. Unless the left is three times or more the size of the right, we can do the simple thing and create a new Bin

  case _ if l.size <= 3 * r.size => Bin(y, l, r)

However, should the left be more than three times the size of the right, we must balance based on the relative sizes of ll and lr, like in scenario five.

  case (Bin(lx, ll, lr), r) if (2*ll.size > lr.size) =>
    Bin(lx, ll, Bin(y, lr, r))
  case (Bin(lx, ll, Bin(lrx, lrl, lrr)), r) =>
    Bin(lrx, Bin(lx, ll, lrl), Bin(y, lrr, r))

This concludes our study of the .insert method and how the ISet is constructed. It should be of no surprise that Foldable is implemented in terms of depth-first search along the left or right, as appropriate. Methods such as .minimum and .maximum are optimal because the data structure already encodes the ordering.

It is worth noting that some typeclass methods cannot be implemented as efficiently as we would like. Consider the signature of Foldable.element

  @typeclass trait Foldable[F[_]] {
    def element[A: Equal](fa: F[A], a: A): Boolean

The obvious implementation for .element is to defer to (almost) binary-search ISet.contains. However, it is not possible because .element provides Equal whereas .contains requires Order.

ISet is unable to provide a Functor for the same reason. In practice this turns out to be a sensible constraint: performing a .map would involve rebuilding the entire structure. It is sensible to convert to some other datatype, such as IList, perform the .map, and convert back. A consequence is that it is not possible to have Traverse[ISet] or Applicative[ISet].

6.8.8 IMap

  sealed abstract class ==>>[A, B] {
    val size: Int = this match {
      case Tip()           => 0
      case Bin(_, _, l, r) => 1 + l.size + r.size
  object ==>> {
    type IMap[A, B] = A ==>> B
    private final case class Tip[A, B]() extends (A ==>> B)
    private final case class Bin[A, B](
      key: A,
      value: B,
      left: A ==>> B,
      right: A ==>> B
    ) extends ==>>[A, B]
    def apply[A: Order, B](x: (A, B)*): A ==>> B = ...
    def empty[A, B]: A ==>> B = Tip[A, B]()
    def singleton[A, B](k: A, x: B): A ==>> B = Bin(k, x, Tip(), Tip())
    def fromFoldable[F[_]: Foldable, A: Order, B](fa: F[(A, B)]): A ==>> B = ...

This is very familiar! Indeed, IMap (an alias to the lightspeed operator ==>>) is another size-balanced tree, but with an extra value: B field in each binary branch, allowing it to store key/value pairs. Only the key type A needs an Order and a suite of convenient methods are provided to allow easy entry updating

  sealed abstract class ==>>[A, B] {
    def adjust(k: A, f: B => B)(implicit o: Order[A]): A ==>> B = ...
    def adjustWithKey(k: A, f: (A, B) => B)(implicit o: Order[A]): A ==>> B = ...

6.8.9 StrictTree and Tree

Both StrictTree and Tree are implementations of a Rose Tree, a tree structure with an unbounded number of branches in every node (unfortunately built from standard library collections for legacy reasons):

  case class StrictTree[A](
    rootLabel: A,
    subForest: Vector[StrictTree[A]]

Tree is a by-need version of StrictTree with convenient constructors

  class Tree[A](
    rootc: Need[A],
    forestc: Need[Stream[Tree[A]]]
  ) {
    def rootLabel = rootc.value
    def subForest = forestc.value
  object Tree {
    object Node {
      def apply[A](root: =>A, forest: =>Stream[Tree[A]]): Tree[A] = ...
    object Leaf {
      def apply[A](root: =>A): Tree[A] = ...

The user of a Rose Tree is expected to manually balance it, which makes it suitable for cases where it is useful to encode domain knowledge of a hierarchy into the data structure. For example, in artificial intelligence, a Rose Tree can be used in clustering algorithms to organise data into a hierarchy of increasingly similar things. It is possible to represent XML documents with a Rose Tree.

If you find yourself working with hierarchical data, consider using a Rose Tree instead of rolling a custom data structure.

6.8.10 FingerTree

Finger trees are generalised sequences with amortised constant cost lookup and logarithmic concatenation. A is the type of data and it may help to ignore V:

  sealed abstract class FingerTree[V, A] {
    def +:(a: A): FingerTree[V, A] = ...
    def :+(a: =>A): FingerTree[V, A] = ...
    def <++>(right: =>FingerTree[V, A]): FingerTree[V, A] = ...
  object FingerTree {
    private class Empty[V, A]() extends FingerTree[V, A]
    private class Single[V, A](v: V, a: =>A) extends FingerTree[V, A]
    private class Deep[V, A](
      v: V,
      left: Finger[V, A],
      spine: =>FingerTree[V, Node[V, A]],
      right: Finger[V, A]
    ) extends FingerTree[V, A]
    sealed abstract class Finger[V, A]
    final case class One[V, A](v: V, a1: A) extends Finger[V, A]
    final case class Two[V, A](v: V, a1: A, a2: A) extends Finger[V, A]
    final case class Three[V, A](v: V, a1: A, a2: A, a3: A) extends Finger[V, A]
    final case class Four[V, A](v: V, a1: A, a2: A, a3: A, a4: A) extends Finger[V, A]
    sealed abstract class Node[V, A]
    private class Node2[V, A](v: V, a1: =>A, a2: =>A) extends Node[V, A]
    private class Node3[V, A](v: V, a1: =>A, a2: =>A, a3: =>A) extends Node[V, A]

Visualising FingerTree as dots, Finger as boxes and Node as boxes within boxes:

Adding elements to the front of a FingerTree with +: is efficient because Deep simply adds the new element to its left finger. If the finger is a Four, we rebuild the spine to take 3 of the elements as a Node3. Adding to the end, :+, is the same but in reverse.

Appending |+| (also <++>) is more efficient than adding one element at a time because the case of two Deep trees can retain the outer branches, rebuilding the spine based on the 16 possible combinations of the two Finger values in the middle.

In the above we skipped over V. Not shown in the ADT description is an implicit measurer: Reducer[A, V] on every element of the ADT.

Reducer is an extension of Monoid that allows for single elements to be added to an M

  class Reducer[C, M: Monoid] {
    def unit(c: C): M
    def snoc(m: M, c: C): M = append(m, unit(c))
    def cons(c: C, m: M): M = append(unit(c), m)

For example, Reducer[A, IList[A]] can provide an efficient .cons

  implicit def reducer[A]: Reducer[A, IList[A]] = new Reducer[A, IList[A]] {
    override def unit(a: A): IList[A] = IList.single(a)
    override def cons(a: A, as: IList[A]): IList[A] = a :: as
  } IndSeq

If we use Int as V, we can get an indexed sequence, where the measure is size, allowing us to perform index-based lookup by comparing the desired index with the size at each branch in the structure:

  final class IndSeq[A](val self: FingerTree[Int, A])
  object IndSeq {
    private implicit def sizer[A]: Reducer[A, Int] = _ => 1
    def apply[A](as: A*): IndSeq[A] = ...

Another use of FingerTree is as an ordered sequence, where the measure stores the largest value contained by each branch: OrdSeq
  final class OrdSeq[A: Order](val self: FingerTree[LastOption[A], A]) {
    def partition(a: A): (OrdSeq[A], OrdSeq[A]) = ...
    def insert(a: A): OrdSeq[A] = ...
    def ++(xs: OrdSeq[A]): OrdSeq[A] = ...
  object OrdSeq {
    private implicit def keyer[A]: Reducer[A, LastOption[A]] = a => Tag(Some(a))
    def apply[A: Order](as: A*): OrdSeq[A] = ...

OrdSeq has no typeclass instances so it is only useful for incrementally building up an ordered sequence, with duplicates. We can access the underlying FingerTree when needed. Cord

The most common use of FingerTree is as an efficient intermediate holder for String representations in Show. Building a single String can be thousands of times faster than the default case class implementation of nested .toString, which builds a String for every layer in the ADT.

  final case class Cord(self: FingerTree[Int, String]) {
    override def toString: String = {
      val sb = new java.lang.StringBuilder(self.measure)
      self.foreach(sb.append) // locally scoped side effect

For example, the Cord[String] instance returns a Three with the string in the middle and quotes on either side

  implicit val show: Show[String] = s => Cord(FingerTree.Three("\"", s, "\""))

Therefore a String renders as it is written in source code

  scala> val s = "foo"
  res: String = foo
  res: Cord = "foo"

6.8.11 Heap Priority Queue

A priority queue is a data structure that allows fast insertion of ordered elements, allowing duplicates, with fast access to the minimum value (highest priority). The structure is not required to store the non-minimal elements in order. A naive implementation of a priority queue could be

  final case class Vip[A] private (val peek: Maybe[A], xs: IList[A]) {
    def push(a: A)(implicit O: Order[A]): Vip[A] = peek match {
      case Maybe.Just(min) if a < min => Vip(a.just, min :: xs)
      case _                          => Vip(peek, a :: xs)
    def pop(implicit O: Order[A]): Maybe[(A, Vip[A])] = peek strengthR reorder
    private def reorder(implicit O: Order[A]): Vip[A] = xs.sorted match {
      case INil()           => Vip(Maybe.empty, IList.empty)
      case ICons(min, rest) => Vip(min.just, rest)
  object Vip {
    def fromList[A: Order](xs: IList[A]): Vip[A] = Vip(Maybe.empty, xs).reorder

This push is a very fast O(1), but reorder (and therefore pop) relies on IList.sorted costing O(n log n).

Scalaz encodes a priority queue, with a tree structure where every node has a value less than its children, called a Heap Priority Queue. Heap has fast push (insert), union, size, pop (uncons) and peek (minimumO) operations:

  sealed abstract class Heap[A] {
    def insert(a: A)(implicit O: Order[A]): Heap[A] = ...
    def +(a: A)(implicit O: Order[A]): Heap[A] = insert(a)
    def union(as: Heap[A])(implicit O: Order[A]): Heap[A] = ...
    def uncons(implicit O: Order[A]): Option[(A, Heap[A])] = minimumO strengthR deleteMin
    def minimumO: Option[A] = ...
    def deleteMin(implicit O: Order[A]): Heap[A] = ...
  object Heap {
    def fromData[F[_]: Foldable, A: Order](as: F[A]): Heap[A] = ...
    private final case class Ranked[A](rank: Int, value: A)
    private final case class Empty[A]() extends Heap[A]
    private final case class NonEmpty[A](
      size: Int,
      tree: Tree[Ranked[A]]
    ) extends Heap[A]

Heap is implemented with a Rose Tree of Ranked values, where the rank is the depth of a subtree, allowing us to depth-balance the tree. We manually maintain the tree so the minimum value is at the top. An advantage of encoding the minimum value in the data structure is that minimumO (also known as peek) is a free lookup:

  def minimumO: Option[A] = this match {
    case Empty()                        => None
    case NonEmpty(_, Tree.Node(min, _)) => Some(min.value)

When inserting a new entry, we compare to the current minimum and replace if the new entry is lower:

  def insert(a: A)(implicit O: Order[A]): Heap[A] = this match {
    case Empty() =>
      NonEmpty(1, Tree.Leaf(Ranked(0, a)))
    case NonEmpty(size, tree @ Tree.Node(min, _)) if a <= min.value =>
      NonEmpty(size + 1, Tree.Node(Ranked(0, a), Stream(tree)))

Insertions of non-minimal values result in an unordered structure in the branches of the minimum. When we encounter two or more subtrees of equal rank, we optimistically put the minimum to the front:

    case NonEmpty(size, Tree.Node(min,
           (t1 @ Tree.Node(Ranked(r1, x1), xs1)) #::
           (t2 @ Tree.Node(Ranked(r2, x2), xs2)) #:: ts)) if r1 == r2 =>
      lazy val t0 = Tree.Leaf(Ranked(0, a))
      val sub =
        if (x1 <= a && x1 <= x2)
          Tree.Node(Ranked(r1 + 1, x1), t0 #:: t2 #:: xs1)
        else if (x2 <= a && x2 <= x1)
          Tree.Node(Ranked(r2 + 1, x2), t0 #:: t1 #:: xs2)
          Tree.Node(Ranked(r1 + 1, a), t1 #:: t2 #:: Stream())
      NonEmpty(size + 1, Tree.Node(Ranked(0, min.value), sub #:: ts))
    case NonEmpty(size,  Tree.Node(min, rest)) =>
      val t0 = Tree.Leaf(Ranked(0, a))
      NonEmpty(size + 1, Tree.Node(Ranked(0, min.value), t0 #:: rest))

Avoiding a full ordering of the tree makes insert very fast, O(1), such that producers adding to the queue are not penalised. However, the consumer pays the cost when calling uncons, with deleteMin costing O(log n) because it must search for the minimum value, and remove it from the tree by rebuilding. That’s fast when compared to the naive implementation.

The union operation also delays ordering allowing it to be O(1).

If the Order[Foo] does not correctly capture the priority we want for the Heap[Foo], we can use Tag and provide a custom Order[Foo @@ Custom] for a Heap[Foo @@ Custom].

6.8.12 Diev (Discrete Intervals)

We can efficiently encode the (unordered) integer values 6, 9, 2, 13, 8, 14, 10, 7, 5 as inclusive intervals [2, 2], [5, 10], [13, 14]. Diev is an efficient encoding of intervals for elements A that have an Enum[A], getting more efficient as the contents become denser.

  sealed abstract class Diev[A] {
    def +(interval: (A, A)): Diev[A]
    def +(value: A): Diev[A]
    def ++(other: Diev[A]): Diev[A]
    def -(interval: (A, A)): Diev[A]
    def -(value: A): Diev[A]
    def --(other: Diev[A]): Diev[A]
    def intervals: Vector[(A, A)]
    def contains(value: A): Boolean
    def contains(interval: (A, A)): Boolean
  object Diev {
    private final case class DieVector[A: Enum](
      intervals: Vector[(A, A)]
    ) extends Diev[A]
    def empty[A: Enum]: Diev[A] = ...
    def fromValuesSeq[A: Enum](values: Seq[A]): Diev[A] = ...
    def fromIntervalsSeq[A: Enum](intervals: Seq[(A, A)]): Diev[A] = ...

When updating the Diev, adjacent intervals are merged (and then ordered) such that there is a unique representation for a given set of values.

  scala> Diev.fromValuesSeq(List(6, 9, 2, 13, 8, 14, 10, 7, 5))
  res: Diev[Int] = ((2,2)(5,10)(13,14))
  scala> Diev.fromValuesSeq(List(6, 9, 2, 13, 8, 14, 10, 7, 5).reverse)
  res: Diev[Int] = ((2,2)(5,10)(13,14))

A great usecase for Diev is for storing time periods. For example, in our TradeTemplate from the previous chapter

  final case class TradeTemplate(
    payments: List[java.time.LocalDate],
    ccy: Option[Currency],
    otc: Option[Boolean]

if we find that the payments are very dense, we may wish to swap to a Diev representation for performance reasons, without any change in our business logic because we used Monoid, not any List specific methods. We would, however, have to provide an Enum[LocalDate], which is an otherwise useful thing to have.

6.8.13 OneAnd

Recall that Foldable is the scalaz equivalent of a collections API and Foldable1 is for non-empty collections. So far we have only seen NonEmptyList to provide a Foldable1. The simple data structure OneAnd wraps any other collection to turn it into a Foldable1:

  final case class OneAnd[F[_], A](head: A, tail: F[A])

NonEmptyList[A] could be an alias to OneAnd[IList, A]. Similarly, we can create non-empty Stream, DList and Tree structures. However it may break ordering and uniqueness characteristics of the underlying structure: a OneAnd[ISet, A] is not a non-empty ISet, it is an ISet with a guaranteed first element that may also be in the ISet.

6.9 Summary

In this chapter we have skimmed over the data types that scalaz has to offer.

It is not necessary to remember everything in this chapter: think of each section as having planted the kernel of an idea in your mind. If, in a few months, you find yourself thinking “I remember reading about a data structure that might be useful here” and you return to these pages, we have succeeded.

The world of functional data structures is an active area of research. Academic publications appear regularly with new approaches to old problems. If you are the kind of person who would like to contribute back to scalaz, finding a functional data structure to implement would be greatly appreciated.

7. Advanced Monads

You have to know things like Advanced Monads in order to be an advanced functional programmer. However, we are developers yearning for a simple life, and our idea of “advanced” is modest. To put it into context: scala.concurrent.Future is more complicated and nuanced than any Monad in this chapter.

In this chapter we will study some of the most important implementations of Monad and explain why Future needlessly complicates an application: we will offer simpler and faster alternatives.

7.1 Always in motion is the Future

The biggest problem with Future is that it eagerly schedules work during construction. As we discovered in the introduction, Future conflates the definition of a program with interpreting it (i.e. running it).

Future is also bad from a performance perspective: every time .flatMap is called, a closure is submitted to an Executor, resulting in unnecessary thread scheduling and context switching. It is not unusual to see 50% of our CPU power dealing with thread scheduling, instead of doing the work. So much so that parallelising work with Future can often make it slower.

Combined, eager evaluation and executor submission means that it is impossible to know when a job started, when it finished, or the sub-tasks that were spawned to calculate the final result. It should not surprise us that performance monitoring “solutions” are a solid earner for the modern day snake oil merchant.

Furthermore, Future.flatMap requires an ExecutionContext to be in implicit scope: users are forced to think about business logic and execution semantics at the same time.

7.2 Effects and Side Effects

If we can’t call side-effecting methods in our business logic, or in Future (or Id, or Either, or Const, etc), when can we write them? The answer is: in a Monad that delays execution until it is interpreted at the application’s entrypoint. We can now refer to I/O and mutation as an effect on the world, captured by the type system, as opposed to having a hidden side-effect.

The simplest implementation of such a Monad is IO, formalising the version we wrote in the introduction:

  final class IO[A](val interpret: () => A)
  object IO {
    def apply[A](a: =>A): IO[A] = new IO(() => a)
    implicit val monad: Monad[IO] = new Monad[IO] {
      def point[A](a: =>A): IO[A] = IO(a)
      def bind[A, B](fa: IO[A])(f: A => IO[B]): IO[B] =

The .interpret method is only called once, in the entrypoint of an application:

  def main(args: Array[String]): Unit = program.interpret()

However, there are two big problems with this simple IO:

  1. it can stack overflow
  2. it doesn’t support parallel computations

Both of these problems will be overcome in this chapter. However, no matter how complicated the internal implementation of a Monad, the principles described here remain true: we’re modularising the definition of a program and its execution, such that we can capture effects in type signatures, allowing us to reason about them, and reuse more code.

7.3 Stack Safety with the Free Monad

On the JVM, every method call adds an entry to the call stack of the Thread, like adding to the front of a List. When the method completes, the method at the head is thrown away. The maximum length of the call stack is determined by the -Xss flag when starting up java. Tail recursive methods are detected by the scala compiler and do not add an entry. If we hit the limit, by calling too many chained methods, we get a StackOverflowError.

Unfortunately, every nested call to our IO’s .flatMap adds another method call to the stack. The easiest way to see this is to repeat an action forever, and see if it survives for longer than a few seconds. We can use .forever, from Apply (a parent of Monad):

  scala> val hello = IO { println("hello") }
  scala> Apply[IO].forever(hello).interpret()
      at ...
      at monadio.IO$$anon$1.$anonfun$bind$1(monadio.scala:18)
      at monadio.IO$$anon$1.$anonfun$bind$1(monadio.scala:18)
      at ...

Scalaz has a typeclass that Monad instances can implement if they are stack safe: BindRec requires a constant stack space for recursive bind:

  @typeclass trait BindRec[F[_]] extends Bind[F] {
    def tailrecM[A, B](f: A => F[A \/ B])(a: A): F[B]
    override def forever[A, B](fa: F[A]): F[B] = ...

We don’t need BindRec for all programs, but it is essential for a general purpose Monad implementation.

The way to achieve stack safety is to convert method calls into references to an ADT, the Free monad:

  sealed abstract class Free[S[_], A]
  object Free {
    private final case class Return[S[_], A](a: A)     extends Free[S, A]
    private final case class Suspend[S[_], A](a: S[A]) extends Free[S, A]
    private final case class Gosub[S[_], A0, B](
      a: Free[S, A0],
      f: A0 => Free[S, B]
    ) extends Free[S, B] { type A = A0 }

The Free monad is named because it can be generated for free for any S[_]. For example, we could set S to be the Drone or Machines algebras from Chapter 3 and generate a data structure representation of our program. We’ll return to why this is useful at the end of this chapter.

7.3.1 Trampoline

Free is more general than we need for now. Setting the algebra S[_] to () => ?, a deferred calculation or thunk, we get Trampoline and can implement a stack safe Monad

  object Free {
    type Trampoline[A] = Free[() => ?, A]
    implicit val trampoline: Monad[Trampoline] with BindRec[Trampoline] =
      new Monad[Trampoline] with BindRec[Trampoline] {
        def point[A](a: =>A): Trampoline[A] = Return(a)
        def bind[A, B](fa: Trampoline[A])(f: A => Trampoline[B]): Trampoline[B] =
          Gosub(fa, f)
        def tailrecM[A, B](f: A => Trampoline[A \/ B])(a: A): Trampoline[B] =
          bind(f(a)) {
            case -\/(a) => tailrecM(f)(a)
            case \/-(b) => point(b)

The Free ADT is a natural data type representation of the Monad interface:

  1. Return represents .point
  2. Gosub represents .bind / .flatMap

When an ADT mirrors the arguments of related functions, it is called a Church encoding, or Continuation Passing Style.

The BindRec implementation, .tailrecM, runs .bind until we get a B. Although this is not technically a @tailrec implementation, it uses constant stack space because each call returns a heap object, with delayed recursion.

Convenient functions are provided to create a Trampoline eagerly (.done) or by-name (.delay). We can also create a Trampoline from a by-name Trampoline (.suspend):

  object Trampoline {
    def done[A](a: A): Trampoline[A]                  = Return(a)
    def delay[A](a: =>A): Trampoline[A]               = suspend(done(a))
    def suspend[A](a: =>Trampoline[A]): Trampoline[A] = unit >> a
    private val unit: Trampoline[Unit] = Suspend(() => done(()))

When we see Trampoline[A] in a codebase we can always mentally substitute it with A, because it is simply adding stack safety to the pure computation. We get the A by interpreting Free, provided by the .run method:

  sealed abstract class Free[S[_], A] {
    def run(implicit ev: Free[S, A] =:= Trampoline[A]): A = ev(this).go(_())
    def go(f: S[Free[S, A]] => Free[S, A])(implicit S: Functor[S]): A = {
      @tailrec def go2(t: Free[S, A]): A = t.resume match {
        case -\/(s) => go2(f(s))
        case \/-(r) => r
    @tailrec def resume(implicit S: Functor[S]): (S[Free[S, A]] \/ A) = this match {
      case Return(a) => \/-(a)
      case Suspend(t) => -\/(
      case Gosub(Return(a), f) => f(a).resume
      case Gosub(Suspend(t), f) => -\/(
      case Gosub(Gosub(a, g), f) => a >>= (z => g(z) >>= f).resume

Take a moment to read through the implementation of resume to understand how this evaluates a single layer of the Free, and that go is running it to completion. The case that is most likely to cause confusion is when we have nested Gosub: apply the inner function g then pass it to the outer one f, it is just function composition.

7.3.2 Example: Stack Safe DList

In the previous chapter we described the data type DList as

  final case class DList[A](f: IList[A] => IList[A]) {
    def toIList: IList[A] = f(IList.empty)
    def ++(as: DList[A]): DList[A] = DList(xs => f(as.f(xs)))

However, the actual implementation looks more like:

  final case class DList[A](f: IList[A] => Trampoline[IList[A]]) {
    def toIList: IList[A] = f(IList.empty).run
    def ++(as: =>DList[A]): DList[A] = DList(xs => suspend(as.f(xs) >>= f))

Instead of applying nested calls to f we use a suspended Trampoline. We interpret the trampoline with .run only when needed, e.g. in toIList. The changes are minimal, but we now have a stack safe DList that can rearrange the concatenation of a large number lists without blowing the stack!

7.3.3 Stack Safe IO

Similarly, our IO can be made stack safe thanks to Trampoline:

  final class IO[A](val tramp: Trampoline[A]) {
    def unsafePerformIO(): A =
  object IO {
    def apply[A](a: =>A): IO[A] = new IO(Trampoline.delay(a))
    implicit val Monad: Monad[IO] with BindRec[IO] =
      new Monad[IO] with BindRec[IO] {
        def point[A](a: =>A): IO[A] = IO(a)
        def bind[A, B](fa: IO[A])(f: A => IO[B]): IO[B] =
          new IO(fa.tramp >>= (a => f(a).tramp))
        def tailrecM[A, B](f: A => IO[A \/ B])(a: A): IO[B] = ...

The interpreter, .unsafePerformIO(), has an intentionally scary name to discourage using it except in the entrypoint of the application.

This time, we don’t get a stack overflow error:

  scala> val hello = IO { println("hello") }
  scala> Apply[IO].forever(hello).unsafePerformIO()

Using a Trampoline typically introduces a performance regression vs a regular reference. It is Free in the sense of freely generated, not free as in beer.

7.4 Monad Transformer Library

Monad transformers are data structures that wrap an underlying value and provide a monadic effect.

For example, in Chapter 2 we used OptionT to let us use F[Option[A]] in a for comprehension as if it was just a F[A]. This gave our program the effect of an optional value. Alternatively, we can get the effect of optionality if we have a MonadPlus.

This subset of data types and extensions to Monad are often referred to as the Monad Transformer Library (MTL), summarised below. In this section, we will explain each of the transformers, why they are useful, and how they work.

Effect Underlying Transformer Typeclass
optionality F[Maybe[A]] MaybeT MonadPlus
errors F[E \/ A] EitherT MonadError
a runtime value A => F[B] ReaderT MonadReader
journal / multitask F[(W, A)] WriterT MonadTell
evolving state S => F[(S, A)] StateT MonadState
keep calm & carry on F[E \&/ A] TheseT  
control flow (A => F[B]) => F[B] ContT  

7.4.1 MonadTrans

Each transformer has the general shape T[F[_], A], providing at least an instance of Monad and Hoist (and therefore MonadTrans):

  @typeclass trait MonadTrans[T[_[_], _]] {
    def liftM[F[_]: Monad, A](a: F[A]): T[F, A]
  @typeclass trait Hoist[F[_[_], _]] extends MonadTrans[F] {
    def hoist[M[_]: Monad, N[_]](f: M ~> N): F[M, ?] ~> F[N, ?]

.liftM lets us create a monad transformer if we have an F[A]. For example, we can create an OptionT[IO, String] by calling .liftM[OptionT] on an IO[String].

.hoist is the same idea, but for natural transformations.

Generally, there are three ways to create a monad transformer:

  • from the underlying, using the transformer’s constructor
  • from a single value A, using .pure from the Monad syntax
  • from an F[A], using .liftM from the MonadTrans syntax

Due to the way that type inference works in Scala, this often means that a complex type parameter must be explicitly written. As a workaround, transformers provide convenient constructors on their companion that are easier to use.

7.4.2 MaybeT

OptionT, MaybeT and LazyOptionT have similar implementations, providing optionality through Option, Maybe and LazyOption, respectively. We will focus on MaybeT to avoid repetition.

  final case class MaybeT[F[_], A](run: F[Maybe[A]])
  object MaybeT {
    def just[F[_]: Applicative, A](v: =>A): MaybeT[F, A] =
    def empty[F[_]: Applicative, A]: MaybeT[F, A] =

providing a MonadPlus

  implicit def monad[F[_]: Monad] = new MonadPlus[MaybeT[F, ?]] {
    def point[A](a: =>A): MaybeT[F, A] = MaybeT.just(a)
    def bind[A, B](fa: MaybeT[F, A])(f: A => MaybeT[F, B]): MaybeT[F, B] =
      MaybeT( >>= (_.cata(f(_).run, Maybe.empty.pure[F])))
    def empty[A]: MaybeT[F, A] = MaybeT.empty
    def plus[A](a: MaybeT[F, A], b: =>MaybeT[F, A]): MaybeT[F, A] = ...

This monad looks fiddly, but it is just delegating everything to the Monad[F] and then re-wrapping with a MaybeT. It is plumbing.

With this monad we can write logic that handles optionality in the F[_] context, rather than carrying around Option or Maybe.

For example, say we are interfacing with a social media website to count the number of stars a user has, and we start with a String that may or may not correspond to a user. We have this algebra:

  trait Twitter[F[_]] {
    def getUser(name: String): F[Maybe[User]]
    def getStars(user: User): F[Int]
  def T[F[_]](implicit t: Twitter[F]): Twitter[F] = t

We need to call getUser followed by getStars. If we use Monad as our context, our function is difficult because we have to handle the Empty case:

  def stars[F[_]: Monad: Twitter](name: String): F[Maybe[Int]] = for {
    maybeUser  <- T.getUser(name)
    maybeStars <- maybeUser.traverse(T.getStars)
  } yield maybeStars

However, if we have a MonadPlus as our context, we can suck Maybe into the F[_] with .orEmpty, and forget about it:

  def stars[F[_]: MonadPlus: Twitter](name: String): F[Int] = for {
    user  <- T.getUser(name) >>= (_.orEmpty[F])
    stars <- T.getStars(user)
  } yield stars

However adding a MonadPlus requirement can cause problems downstream if the context does not have one. The solution is to either change the context of the program to MaybeT[F, ?] (lifting the Monad[F] into a MonadPlus), or to explicitly use MaybeT in the return type, at the cost of slightly more code:

  def stars[F[_]: Monad: Twitter](name: String): MaybeT[F, Int] = for {
    user  <- MaybeT(T.getUser(name))
    stars <- T.getStars(user).liftM[MaybeT]
  } yield stars

The decision to require a more powerful Monad vs returning a transformer is something that each team can decide for themselves based on the interpreters that they plan on using for their program.

7.4.3 EitherT

An optional value is a special case of a value that may be an error, but we don’t know anything about the error. EitherT (and the lazy variant LazyEitherT) allows us to use any type we want as the error value, providing contextual information about what went wrong.

Where \/ and Validation is the FP equivalent of a checked exception, EitherT makes it convenient to both create and ignore errors until something can be done about it.

EitherT is a wrapper around an F[A \/ B]

  final case class EitherT[F[_], A, B](run: F[A \/ B])
  object EitherT {
    def either[F[_]: Applicative, A, B](d: A \/ B): EitherT[F, A, B] = ...
    def leftT[F[_]: Functor, A, B](fa: F[A]): EitherT[F, A, B] = ...
    def rightT[F[_]: Functor, A, B](fb: F[B]): EitherT[F, A, B] = ...
    def pureLeft[F[_]: Applicative, A, B](a: A): EitherT[F, A, B] = ...
    def pure[F[_]: Applicative, A, B](b: B): EitherT[F, A, B] = ...

The Monad is a MonadError

  @typeclass trait MonadError[F[_], E] extends Monad[F] {
    def raiseError[A](e: E): F[A]
    def handleError[A](fa: F[A])(f: E => F[A]): F[A]
    def emap[A, B](fa: F[A])(f: A => S \/ B): F[B] =
      bind(fa)(a => f(a).fold(raiseError(_), pure(_)))

.raiseError and .handleError are self-descriptive: the equivalent of throw and catch an exception, respectively.

.emap, either map, is for functions that could fail and is very useful for writing decoders in terms of existing ones, much as we used .contramap to define new encoders in terms of existing ones. For example, say we have an XML decoder like

  @typeclass trait XDecoder[A] {
    def fromXml(x: Xml): String \/ A
  object XDecoder {
    implicit val monad: MonadError[String \/ ?, String] = ...
    implicit val string: XDecoder[String] = ...

we can define a decoder for Char in terms of a String decoder

  implicit val char: XDecoder[Char] = XDecoder[String].emap { s =>
    if (s.length == 1) s(0).right
    else s"not a char: $s".left

The MonadError for EitherT is:

  implicit def monad[F[_]: Monad, E] = new MonadError[EitherT[F, E, ?], E] {
    def monad[F[_]: Monad, E] = new MonadError[EitherT[F, E, ?], E] {
    def bind[A, B](fa: EitherT[F, E, A])
                  (f: A => EitherT[F, E, B]): EitherT[F, E, B] =
      EitherT( >>= (_.fold(_.left[B].pure[F], b => f(b).run)))
    def point[A](a: =>A): EitherT[F, E, A] = EitherT.pure(a)
    def raiseError[A](e: E): EitherT[F, E, A] = EitherT.pureLeft(e)
    def handleError[A](fa: EitherT[F, E, A])
                      (f: E => EitherT[F, E, A]): EitherT[F, E, A] =
      EitherT( >>= {
        case -\/(e) => f(e).run
        case right => right.pure[F]

It should be of no surprise that we can rewrite the MonadPlus example with MonadError, inserting informative error messages:

  def stars[F[_]: Twitter](name: String)
                          (implicit F: MonadError[F, String]): F[Int] = for {
    user  <- T.getUser(name) >>= (_.orError(s"user '$name' not found")(F))
    stars <- T.getStars(user)
  } yield stars

where .orError is a convenience method on Maybe

  sealed abstract class Maybe[A] {
    def orError[F[_], E](e: E)(implicit F: MonadError[F, E]): F[A] =
      cata(F.point(_), F.raiseError(e))

The version using EitherT directly looks like

  def stars[F[_]: Monad: Twitter](name: String): EitherT[F, String, Int] = for {
    user <- EitherT(T.getUser(name).map(_ \/> s"user '$name' not found"))
    stars <- EitherT.rightT(T.getStars(user))
  } yield stars

The simplest instance of MonadError is for \/, perfect for testing business logic that requires a MonadError. For example,

  final class MockTwitter extends Twitter[String \/ ?] {
    def getUser(name: String): String \/ Maybe[User] =
      if (name.contains(" ")) Maybe.empty.right
      else if (name === "wobble") "connection error".left
      else User(name).just.right
    def getStars(user: User): String \/ Int =
      if ("w")) 10.right
      else "stars have been replaced by hearts".left

Our unit tests for .stars might cover these cases:

  scala> stars("wibble")
  scala> stars("wobble")
  -\/(connection error)
  scala> stars("i'm a fish")
  -\/(user 'i'm a fish' not found)
  scala> stars("fommil")
  -\/(stars have been replaced by hearts)

As we’ve now seen several times, we can focus on testing the pure business logic without distraction. Choosing an error type

The community is undecided on the best strategy for the error type E in MonadError.

One school of thought says that we should pick something general, like a String. The other school says that an application should have an ADT of errors, allowing different errors to be reported or handled differently. An unprincipled gang prefers using Throwable for maximum JVM compatibility.

There are two problems with an ADT of errors on the application level:

  • it is very awkward to create a new error. One file becomes a monolithic repository of errors, aggregating the ADTs of individual subsystems.
  • no matter how granular the errors are, the resolution is often the same: log it and try it again, or give up. We don’t need an ADT for this.

An error ADT is of value if every entry allows a different kind of recovery to be performed.

A compromise between an error ADT and a String is an intermediary format. JSON is a good choice as it can be understood by most logging and monitoring frameworks.

A problem with not having a stacktrace is that it can be hard to localise which piece of code was the source of an error. With sourcecode by Li Haoyi, we can include contextual information as metadata in our errors:

  final case class Meta(fqn: String, file: String, line: Int)
  object Meta {
    implicit def gen(implicit fqn: sourcecode.FullName,
                              file: sourcecode.File,
                              line: sourcecode.Line): Meta =
      new Meta(fqn.value, file.value, line.value)
  final case class Err(msg: String)(implicit val meta: Meta)

Although Err is referentially transparent, the implicit construction of a Meta does not appear to be referentially transparent from a natural reading: two calls to Meta.gen (invoked implicitly when creating an Err) will produce different values because the location in the source code impacts the returned value:

  scala> println(Err("hello world").meta)
  scala> println(Err("hello world").meta)

To understand this, we have to appreciate that sourcecode.* methods are macros that are generating source code for us. If we were to write the above explicitly it is clear what is happening:

  scala> println(Err("hello world")(Meta("com.acme", "<console>", 10)).meta)
  scala> println(Err("hello world")(Meta("com.acme", "<console>", 11)).meta)

Yes, we’ve made a deal with the macro devil, but we could also write the Meta manually and have it go out of date quicker than our documentation. IO and Throwable

IO does not have a MonadError but instead implements something similar to throw, catch and finally for Throwable errors:

  final class IO[A](val tramp: Trampoline[A]) {
    // catch
    def except(f: Throwable => IO[A]): IO[A] = ...
    // finally
    def ensuring[B](f: IO[B]): IO[A] = ...
    def onException[B](f: IO[B]): IO[A] = ...
  object IO {
    // throw
    def throwIO[A](e: Throwable): IO[A] = IO(throw e)

It is good to avoid using keywords where possible, as it means we have to remember less caveats of the language. If we need to interact with a legacy API with a predictable exception, like a string parser, we can use Maybe.attempt or \/.attempt and convert the non referentially transparent Throwable into a descriptive String.

7.4.4 ReaderT

The reader monad wraps A => F[B] allowing a program F[B] to depend on a runtime value A. For those familiar with dependency injection, the reader monad is the FP equivalent of Spring or Guice’s @Inject, without the XML and reflection.

ReaderT is just an alias to another more generally useful data type named after the mathematician Heinrich Kleisli.

  type ReaderT[F[_], A, B] = Kleisli[F, A, B]
  final case class Kleisli[F[_], A, B](run: A => F[B]) {
    def dimap[C, D](f: C => A, g: B => D)(implicit F: Functor[F]): Kleisli[F, C, D] =
      Kleisli(c => run(f(c)).map(g))
    def >=>[C](k: Kleisli[F, B, C])(implicit F: Bind[F]): Kleisli[F, A, C] = ...
    def >==>[C](k: B => F[C])(implicit F: Bind[F]): Kleisli[F, A, C] = this >=> Kleisli(k)
  object Kleisli {
    implicit def kleisliFn[F[_], A, B](k: Kleisli[F, A, B]): A => F[B] =

An implicit conversion on the companion allows us to use a Kleisli in place of a function, so we can provide it as the parameter to a monad’s .bind, or >>=.

The most common use for ReaderT is to provide environment information to a program. In drone-dynamic-agents we need access to the user’s Oauth 2.0 Refresh Token to be able to contact Google. The obvious thing is to load the RefreshTokens from disk on startup, and make every method take an implicit tokens: RefreshToken. In fact, this is such a common requirement that Martin Odersky has proposed implicit functions.

A better solution is for our program to have an algebra that provides the configuration when needed, e.g.

  trait ConfigReader[F[_]] {
    def token: F[RefreshToken]

We have reinvented MonadReader, the typeclass associated to ReaderT, where .ask is the same as our .token, and S is RefreshToken:

  @typeclass trait MonadReader[F[_], S] extends Monad[F] {
    def ask: F[S]
    def local[A](f: S => S)(fa: F[A]): F[A]

with the implementation

  implicit def monad[F[_]: Monad, R] = new MonadReader[Kleisli[F, R, ?], R] {
    def point[A](a: =>A): Kleisli[F, R, A] = Kleisli(_ => F.point(a))
    def bind[A, B](fa: Kleisli[F, R, A])(f: A => Kleisli[F, R, B]) =
      Kleisli(a => Monad[F].bind(
    def ask: Kleisli[F, R, R] = Kleisli(_.pure[F])
    def local[A](f: R => R)(fa: Kleisli[F, R, A]): Kleisli[F, R, A] =
      Kleisli(f andThen

A law of MonadReader is that the S cannot change between invocations, i.e. ask >> ask === ask. For our usecase, this is to say that the configuration is read once. If we decide later that we want to reload configuration every time we need it, e.g. allowing us to change the token without restarting the application, we can reintroduce ConfigReader which has no such law.

In our OAuth 2.0 implementation we could first move the Monad evidence onto the methods:

  def bearer(refresh: RefreshToken)(implicit F: Monad[F]): F[BearerToken] =
    for { ...

and then refactor the refresh parameter to be part of the Monad

  def bearer(implicit F: MonadReader[F, RefreshToken]): F[BearerToken] =
    for {
      refresh <- F.ask

Fundamentally, any parameter can be moved into the MonadReader. This is of most value to your immediate caller when they simply want to thread through this information from above. With ReaderT, we can reserve implicit parameter blocks entirely for the use of typeclasses, reducing the mental burden of using Scala.

The other method in MonadReader is .local

  def local[A](f: S => S)(fa: F[A]): F[A]

We can change S and run a program fa within that local context, returning to the original S. A use case for .local is to generate a “stack trace” that makes sense to our domain. giving us nested logging! Leaning on the Meta data structure from the previous section, we define a function to checkpoint:

  def traced[A](fa: F[A])(implicit F: MonadReader[F, IList[Meta]]): F[A] =
    F.local(Meta.gen :: _)(fa)

and we can use it to wrap functions that operate in this context.

  def foo: F[Foo] = traced(getBar) >>= barToFoo

automatically passing through anything that is not explicitly traced. A compiler plugin or macro could do the opposite, opting everything in by default.

If we access .ask we can see the breadcrumb trail of exactly how we were called, without the distraction of bytecode implementation details. A referentially transparent stacktrace!

A defensive programmer may wish to truncate the IList[Meta] at a certain length to avoid the equivalent of a stack overflow. Indeed, a more appropriate data structure is Dequeue.

.local can also be used to keep track of contextual information that is directly relevant to the task at hand, like the number of spaces that must indent a line when pretty printing a human readable file format, bumping it by two spaces when we enter a nested structure.

Finally, if we cannot request a MonadReader because our application does not provide one, we can always return a ReaderT

  def bearer(implicit F: Monad[F]): ReaderT[F, RefreshToken, BearerToken] =
    ReaderT( token => for {

If a caller receives a ReaderT, and they have the token parameter to hand, they can call and get back an F[BearerToken].

Admittedly, since we don’t have many callers, we should just revert to a regular function parameter. MonadReader is of most use when:

  1. we may wish to refactor the code later to reload config
  2. the value is not needed by intermediate callers
  3. or, we want to locally scope some variable

In a nutshell, dotty can keep its implicit functions… we already have ReaderT and MonadReader.

One last example. Monad transformers typically provide specialised Monad instances if their underlying type has one. So, for example, ReaderT has a MonadError, MonadPlus, etc if the underlying has one. Decoder typeclasses tend to have a signature that looks like A => F[B], recall

  @typeclass trait XDecoder[A] {
    def fromXml(x: Xml): String \/ A

which has a single method of signature XNode => String \/ A, isomorphic to ReaderT[String \/ ?, Xml, A]. We can formalise this relationship with an Isomorphism. It’s easier to read by introducing type aliases

  type Out[a] = String \/ a
  type RT[a] = ReaderT[Out, Xml, a]
  val isoReaderT: XDecoder <~> RT =
    new IsoFunctorTemplate[XDecoder, RT] {
      def from[A](fa: RT[A]): XDecoder[A] =
      def to[A](fa: XDecoder[A]): RT[A] = ReaderT[Out, Xml, A](fa.fromXml)

Now our XDecoder has access to all the typeclasses that ReaderT has. The typeclass we need is MonadError[Decoder, String]

  implicit val monad: MonadError[XDecoder, String] = MonadError.fromIso(isoReaderT)

which we know to be useful for defining new decoders in terms of existing ones.

7.4.5 WriterT

The opposite to reading is writing. The WriterT monad transformer is typically for writing to a journal.

  final case class WriterT[F[_], W, A](run: F[(W, A)])
  object WriterT {
    def put[F[_]: Functor, W, A](value: F[A])(w: W): WriterT[F, W, A] = ...
    def putWith[F[_]: Functor, W, A](value: F[A])(w: A => W): WriterT[F, W, A] = ...

The wrapped type is F[(W, A)] with the journal accumulated in W.

There is not just one associated monad, but two! MonadTell and MonadListen

  @typeclass trait MonadTell[F[_], W] extends Monad[F] {
    def writer[A](w: W, v: A): F[A]
    def tell(w: W): F[Unit] = ...
    def :++>[A](fa: F[A])(w: =>W): F[A] = ...
    def :++>>[A](fa: F[A])(f: A => W): F[A] = ...
  @typeclass trait MonadListen[F[_], W] extends MonadTell[F, W] {
    def listen[A](fa: F[A]): F[(A, W)]
    def written[A](fa: F[A]): F[W] = ...

MonadTell is for writing to the journal and MonadListen is to recover it. The WriterT implementation is

  implicit def monad[F[_]: Monad, W: Monoid] = new MonadListen[WriterT[F, W, ?], W] {
    def point[A](a: =>A) = WriterT((Monoid[W].zero, a).point)
    def bind[A, B](fa: WriterT[F, W, A])(f: A => WriterT[F, W, B]) = WriterT( >>= { case (wa, a) => f(a) { case (wb, b) => (wa |+| wb, b) } })
    def writer[A](w: W, v: A) = WriterT((w -> v).point)
    def listen[A](fa: WriterT[F, W, A]) = WriterT( { case (w, a) => (w, (a, w)) })

The most obvious example is to use MonadWriter for logging, or audit reporting. Reusing Meta from our error reporting we could imagine creating a log structure like

  sealed trait Log
  final case class Debug(msg: String)(implicit m: Meta)   extends Log
  final case class Info(msg: String)(implicit m: Meta)    extends Log
  final case class Warning(msg: String)(implicit m: Meta) extends Log

and use Dequeue[Log] as our journal type. We could change our OAuth2 authenticate method to

  def debug(msg: String)(implicit m: Meta): Dequeue[Log] = Dequeue(Debug(msg))
  def authenticate: F[CodeToken] =
    for {
      callback <- user.start :++> debug("started the webserver")
      params   = AuthRequest(callback, config.scope, config.clientId)
      url      = config.auth.withQuery(params.toUrlQuery)
      _        <- :++> debug(s"user visiting $url")
      code     <- user.stop :++> debug("stopped the webserver")
    } yield code

We could even combine this with the ReaderT traces and get structured logs.

The caller can recover the logs with .written and do something with them.

However, there is a strong argument that logging deserves its own algebra. The log level is often needed at the point of creation for performance reasons and writing out the logs is typically managed at the application level rather than something each component needs to be concerned about.

The W in WriterT has a Monoid, allowing us to journal any kind of monoidic calculation as a secondary value along with our primary program. For example, counting the number of times we do something, building up an explanation of a calculation, or building up a TradeTemplate for a new trade while we price it.

A popular specialisation of WriterT is when the monad is Id, meaning the underlying run value is just a simple tuple (W, A).

  type Writer[W, A] = WriterT[Id, W, A]
  object WriterT {
    def writer[W, A](v: (W, A)): Writer[W, A] = WriterT[Id, W, A](v)
    def tell[W](w: W): Writer[W, Unit] = WriterT((w, ()))
  final implicit class WriterOps[A](self: A) {
    def set[W](w: W): Writer[W, A] = WriterT(w -> self)
    def tell: Writer[A, Unit] = WriterT.tell(self)

which allows us to let any value carry around a secondary monoidal calculation, without needing a context F[_].

In a nutshell, WriterT / MonadTell is how to multi-task in FP.

7.4.6 StateT

StateT lets us .put, .get and .modify a value that is handled by the monadic context. It is the FP replacement of var.

If we were to write an impure method that has access to some mutable state, held in a var, it might have the signature () => F[A] and return a different value on every call, breaking referential transparency. With pure FP the function takes the state as input and returns the updated state as output, which is why the underlying type of StateT is S => F[(S, A)].

The associated monad is MonadState

  @typeclass trait MonadState[F[_], S] extends Monad[F] {
    def put(s: S): F[Unit]
    def get: F[S]
    def modify(f: S => S): F[Unit] = get >>= (s => put(f(s)))

StateT is implemented slightly differently than the monad transformers we have studied so far. Instead of being a case class it is an ADT with two members:

  sealed abstract class StateT[F[_], S, A]
  object StateT {
    def apply[F[_], S, A](f: S => F[(S, A)]): StateT[F, S, A] = Point(f)
    private final case class Point[F[_], S, A](
      run: S => F[(S, A)]
    ) extends StateT[F, S, A]
    private final case class FlatMap[F[_], S, A, B](
      a: StateT[F, S, A],
      f: (S, A) => StateT[F, S, B]
    ) extends StateT[F, S, B]

which are a specialised form of Trampoline, giving us stack safety when we want to recover the underlying data structure, .run:

  sealed abstract class StateT[F[_], S, A] {
    def run(initial: S)(implicit F: Monad[F]): F[(S, A)] = this match {
      case Point(f) => f(initial)
      case FlatMap(Point(f), g) =>
        f(initial) >>= { case (s, x) => g(s, x).run(s) }
      case FlatMap(FlatMap(f, g), h) =>
        FlatMap(f, (s, x) => FlatMap(g(s, x), h)).run(initial)

StateT can straightforwardly implement MonadState with its ADT:

  implicit def monad[F[_]: Applicative, S] = new MonadState[StateT[F, S, ?], S] {
    def point[A](a: =>A) = Point(s => (s, a).point[F])
    def bind[A, B](fa: StateT[F, S, A])(f: A => StateT[F, S, B]) =
      FlatMap(fa, (_, a: A) => f(a))
    def get       = Point(s => (s, s).point[F])
    def put(s: S) = Point(_ => (s, ()).point[F])

With .pure mirrored on the companion as .stateT:

  object StateT {
    def stateT[F[_]: Applicative, S, A](a: A): StateT[F, S, A] = ...

and MonadTrans.liftM providing the F[A] => StateT[F, S, A] constructor as usual.

A common variant of StateT is when F = Id, giving the underlying type signature S => (S, A). Scalaz provides a type alias and convenience functions for interacting with the State monad transformer directly, and mirroring MonadState:

  type State[a] = StateT[Id, a]
  object State {
    def apply[S, A](f: S => (S, A)): State[S, A] = StateT[Id, S, A](f)
    def state[S, A](a: A): State[S, A] = State((_, a))
    def get[S]: State[S, S] = State(s => (s, s))
    def put[S](s: S): State[S, Unit] = State(_ => (s, ()))
    def modify[S](f: S => S): State[S, Unit] = ...

For an example we can return to the business logic tests of drone-dynamic-agents. Recall from Chapter 3 that we created Mutable as test interpreters for our application and we stored the number of started and stoped nodes in var.

  class Mutable(state: WorldView) {
    var started, stopped: Int = 0
    implicit val drone: Drone[Id] = new Drone[Id] { ... }
    implicit val machines: Machines[Id] = new Machines[Id] { ... }
    val program = new DynAgents[Id]

We now know that we can write a much better test simulator with State. We’ll take the opportunity to upgrade the accuracy of the simulation at the same time. Recall that a core domain object is our application’s view of the world:

  final case class WorldView(
    backlog: Int,
    agents: Int,
    managed: NonEmptyList[MachineNode],
    alive: Map[MachineNode, Instant],
    pending: Map[MachineNode, Instant],
    time: Instant

Since we’re writing a simulation of the world for our tests, we can create a data type that captures the ground truth of everything

  final case class World(
    backlog: Int,
    agents: Int,
    managed: NonEmptyList[MachineNode],
    alive: Map[MachineNode, Instant],
    started: Set[MachineNode],
    stopped: Set[MachineNode],
    time: Instant

The key difference being that the started and stopped nodes can be separated out. Our interpreter can be implemented in terms of State[World, a] and we can write our tests to assert on what both the World and WorldView looks like after the business logic has run.

The interpreters, which are mocking out contacting external Drone and Google services, may be implemented like this:

  import State.{ get, modify }
  object StateImpl {
    type F[a] = State[World, a]
    implicit val drone: Drone[F] = new Drone[F] {
      def getBacklog: F[Int] =
      def getAgents: F[Int]  =
    implicit val machines: Machines[F] = new Machines[F] {
      def getAlive: F[Map[MachineNode, Instant]]   =
      def getManaged: F[NonEmptyList[MachineNode]] =
      def getTime: F[Instant]                      =
      def start(node: MachineNode): F[Unit] =
        modify(w => w.copy(started = w.started + node))
      def stop(node: MachineNode): F[Unit] =
        modify(w => w.copy(stopped = w.stopped + node))
    val program: DynAgents[F] = new DynAgents[F]

and we can rewrite our tests to follow a convention where:

  • world1 is the state of the world before running the program
  • view1 is the application’s belief about the world
  • world2 is the state of the world after running the program
  • view2 is the application’s belief after running the program

For example,

  it should "request agents when needed" in {
    val world1          = World(5, 0, managed, Map(), Set(), Set(), time1)
    val view1           = WorldView(5, 0, managed, Map(), Map(), time1)
    val (world2, view2) = StateImpl.program.act(view1).run(world1)
    view2.shouldBe(view1.copy(pending = Map(node1 -> time1)))

We would be forgiven for looking back to our business logic loop

  state = initial()
  while True:
    state = update(state)
    state = act(state)

and use StateT to manage the state. However, our DynAgents business logic requires only Applicative and we would be violating the Rule of Least Power to require the more powerful MonadState. It is therefore entirely reasonable to handle the state manually by passing it in to update and act.

7.4.7 IndexedStateT

The code that we have studied thus far is not how scalaz implements StateT. Instead, a type alias points to IndexedStateT

  type StateT[F[_], S, A] = IndexedStateT[F, S, S, A]

The implementation of IndexedStateT is much as we have studied, with an extra type parameter allowing the input state S1 and output state S2 to differ:

  sealed abstract class IndexedStateT[F[_], -S1, S2, A] {
    def run(initial: S1)(implicit F: Bind[F]): F[(S2, A)] = ...
  object IndexedStateT {
    def apply[F[_], S1, S2, A](
      f: S1 => F[(S2, A)]
    ): IndexedStateT[F, S1, S2, A] = Wrap(f)
    private final case class Wrap[F[_], S1, S2, A](
      run: S1 => F[(S2, A)]
    ) extends IndexedStateT[F, S1, S2, A]
    private final case class FlatMap[F[_], S1, S2, S3, A, B](
      a: IndexedStateT[F, S1, S2, A],
      f: (S2, A) => IndexedStateT[F, S2, S3, B]
    ) extends IndexedStateT[F, S1, S3, B]

IndexedStateT does not have a MonadState when S1 ! S2=, although it has a Monad.

The following example is adapted from Index your State by Vincent Marquez. Consider the scenario where we must design an algebraic interface to access a key: Int to value: String lookup. This may have a networked implementation and the order of calls is essential. Our first attempt at the API may look something like:

  trait Cache[F[_]] {
    def read(k: Int): F[Maybe[String]]
    def lock: F[Unit]
    def update(k: Int, v: String): F[Unit]
    def commit: F[Unit]

with runtime errors if .update or .commit is called without a .lock. A more complex design may involve multiple traits and a custom DSL that nobody remembers how to use.

Instead, we can use IndexedStateT to require that the caller is in the correct state. First we define our possible states as an ADT

  sealed abstract class Status
  final case class Ready()                          extends Status
  final case class Locked(on: ISet[Int])            extends Status
  final case class Updated(values: Int ==>> String) extends Status

and then revisit our algebra

  trait Cache[M[_]] {
    type F[in, out, a] = IndexedStateT[M, in, out, a]
    def read(k: Int): F[Ready, Ready, Maybe[String]]
    def readLocked(k: Int): F[Locked, Locked, Maybe[String]]
    def readUncommitted(k: Int): F[Updated, Updated, Maybe[String]]
    def lock: F[Ready, Locked, Unit]
    def update(k: Int, v: String): F[Locked, Updated, Unit]
    def commit: F[Updated, Ready, Unit]

which will give a compiletime error if we try to .update without a .lock

  for {
        a1 <-
        _  <- C.update(13, "wibble")
        _  <- C.commit
      } yield a1
  [error]  found   : IndexedStateT[M,Locked,Ready,Maybe[String]]
  [error]  required: IndexedStateT[M,Ready,?,?]
  [error]       _  <- C.update(13, "wibble")
  [error]          ^

but allowing us to construct functions that can be composed by explicitly including their state:

  def wibbleise[M[_]: Monad](C: Cache[M]): F[Ready, Ready, String] =
    for {
      _  <- C.lock
      a1 <- C.readLocked(13)
      a2 = a1.cata(_ + "'", "wibble")
      _  <- C.update(13, a2)
      _  <- C.commit
    } yield a2

7.4.8 IndexedReaderWriterStateT

Those wanting to have a combination of ReaderT, WriterT and IndexedStateT will not be disappointed. The transformer IndexedReaderWriterStateT wraps (R, S1) => F[(W, A, S2)] with R having Reader semantics, W for monoidic writes, and the S parameters for indexed state updates.

  sealed abstract class IndexedReaderWriterStateT[F[_], -R, W, -S1, S2, A] {
    def run(r: R, s: S1)(implicit F: Monad[F]): F[(W, A, S2)] = ...
  object IndexedReaderWriterStateT {
    def apply[F[_], R, W, S1, S2, A](f: (R, S1) => F[(W, A, S2)]) = ...
  type ReaderWriterStateT[F[_], -R, W, S, A] = IndexedReaderWriterStateT[F, R, W, S, S, A]
  object ReaderWriterStateT {
    def apply[F[_], R, W, S, A](f: (R, S) => F[(W, A, S)]) = ...

Abbreviations are provided because otherwise, let’s be honest, these types are so long they look like they are part of a J2EE API:

  type IRWST[F[_], -R, W, -S1, S2, A] = IndexedReaderWriterStateT[F, R, W, S1, S2, A]
  val IRWST = IndexedReaderWriterStateT
  type RWST[F[_], -R, W, S, A] = ReaderWriterStateT[F, R, W, S, A]
  val RWST = ReaderWriterStateT

IRWST is a more efficient implementation than a manually created transformer stack of ReaderT[WriterT[IndexedStateT[F, ...], ...], ...].

7.4.9 TheseT

TheseT allows errors to either abort the calculation or to be accumulated if there is some partial success. Hence keep calm and carry on.

The underlying data type is F[A \&/ B] with A being the error type, requiring a Semigroup to enable the accumulation of errors.

  final case class TheseT[F[_], A, B](run: F[A \&/ B])
  object TheseT {
    def `this`[F[_]: Functor, A, B](a: F[A]): TheseT[F, A, B] = ...
    def that[F[_]: Functor, A, B](b: F[B]): TheseT[F, A, B] = ...
    def both[F[_]: Functor, A, B](ab: F[(A, B)]): TheseT[F, A, B] = ...
    implicit def monad[F[_]: Monad, A: Semigroup] = new Monad[TheseT[F, A, ?]] {
      def bind[B, C](fa: TheseT[F, A, B])(f: B => TheseT[F, A, C]) =
        TheseT( >>= {
          case This(a) => a.wrapThis[C].point[F]
          case That(b) => f(b).run
          case Both(a, b) =>
            f(b) {
              case This(a_)     => (a |+| a_).wrapThis[C]
              case That(c_)     => Both(a, c_)
              case Both(a_, c_) => Both(a |+| a_, c_)
      def point[B](b: =>B) = TheseT(b.wrapThat.point[F])

There is no special monad associated with TheseT, it is just a regular Monad. If we wish to abort a calculation we can return a This value, but we accumulate errors when we return a Both which also contains a successful part of the calculation.

TheseT can also be thought of from a different angle: A does not need to be an error. Similarly to WriterT, the A may be a secondary calculation that we are computing along with the primary calculation B. TheseT allows early exit when something special about A demands it, like when Charlie Bucket found the last golden ticket (A) he threw away his chocolate bar (B).

7.4.10 ContT

Continuation Passing Style (CPS) is a style of programming where functions never return, instead continuing to the next computation. CPS is popular in Javascript and Lisp as they allow non-blocking I/O via callbacks when data is available. A direct translation of the pattern into impure Scala looks like

  def foo[I, A](input: I)(next: A => Unit): Unit = next(doSomeStuff(input))

We can make this pure by introducing an F[_] context

  def foo[F[_], I, A](input: I)(next: A => F[Unit]): F[Unit]

and refactor to return a function for the provided input

  def foo[F[_], I, A](input: I): (A => F[Unit]) => F[Unit]

ContT is just a container for this signature, with a Monad instance

  final case class ContT[F[_], B, A](_run: (A => F[B]) => F[B]) {
    def run(f: A => F[B]): F[B] = _run(f)
  object IndexedContT {
    implicit def monad[F[_], B] = new Monad[ContT[F, B, ?]] {
      def point[A](a: =>A) = ContT(_(a))
      def bind[A, C](fa: ContT[F, B, A])(f: A => ContT[F, B, C]) =
        ContT(c_fb => => f(a).run(c_fb)))

and convenient syntax to create a ContT from a monadic value:

  implicit class ContTOps[F[_]: Monad, A](self: F[A]) {
    def cps[B]: ContT[F, B, A] = ContT(a_fb => self >>= a_fb)

However, the simple callback use of continuations brings nothing to pure functional programming because we already know how to sequence non-blocking, potentially distributed, computations: that’s what Monad is for and we can do this with .bind or a Kleisli arrow. To see why continuations are useful we need to consider a more complex example under a rigid design constraint. Control Flow

Say we have modularised our application into components that can perform I/O, with each component owned by a different development team:

  final case class A0()
  final case class A1()
  final case class A2()
  final case class A3()
  final case class A4()
  def bar0(a4: A4): IO[A0] = ...
  def bar2(a1: A1): IO[A2] = ...
  def bar3(a2: A2): IO[A3] = ...
  def bar4(a3: A3): IO[A4] = ...

Our goal is to produce an A0 given an A1. Whereas Javascript and Lisp would reach for continuations to solve this problem (because the I/O could block) we can just chain the functions

  def simple(a: A1): IO[A0] = bar2(a) >>= bar3 >>= bar4 >>= bar0

We can lift .simple into its continuation form by using the convenient .cps syntax and a little bit of extra boilerplate for each step:

  def foo1(a: A1): ContT[IO, A0, A2] = bar2(a).cps
  def foo2(a: A2): ContT[IO, A0, A3] = bar3(a).cps
  def foo3(a: A3): ContT[IO, A0, A4] = bar4(a).cps
  def flow(a: A1): IO[A0]  = (foo1(a) >>= foo2 >>= foo3).run(bar0)

So what does this buy us? Firstly, it’s worth noting that the control flow of this application is left to right

What if we are the authors of foo2 and we want to post-process the a0 that we receive from the right (downstream), i.e. we want to split our foo2 into foo2a and foo2b

Let’s add the constraint that we cannot change the definition of flow or bar0, perhaps it is not our code and is defined by the framework we are using.

It is not possible to process the output of a0 by modifying any of the remaining barX methods. However, with ContT we can modify foo2 to process the result of the next continuation:

Which can be defined with

  def foo2(a: A2): ContT[IO, A0, A3] = ContT { next =>
    for {
      a3  <- bar3(a)
      a0  <- next(a3)
    } yield process(a0)

We are not limited to .map over the return value, we can .bind into another control flow turning the linear flow into a graph!

  def elsewhere: ContT[IO, A0, A4] = ???
  def foo2(a: A2): ContT[IO, A0, A3] = ContT { next =>
    for {
      a3  <- bar3(a)
      a0  <- next(a3)
      a0_ <- if (check(a0)) a0.pure[IO]
    } yield a0_

Or we can stay within the original flow and retry everything downstream

  def foo2(a: A2): ContT[IO, A0, A3] = ContT { next =>
    for {
      a3  <- bar3(a)
      a0  <- next(a3)
      a0_ <- if (check(a0)) a0.pure[IO]
             else next(a3)
    } yield a0_

This is just one retry, not an infinite loop. For example, we might want downstream to reconfirm a potentially dangerous action.

Finally, we can perform actions that are specific to the context of the ContT, in this case IO which lets us do error handling and resource cleanup:

  def foo2(a: A2): ContT[IO, A0, A3] = bar3(a).ensuring(cleanup).cps When to Order Spaghetti

It is not an accident that these diagrams look like spaghetti, that’s just what happens when we start messing with control flow. All the mechanisms we’ve discussed in this section are simple to implement directly if we can edit the definition of flow, therefore we do not typically need to use ContT.

However, if we are designing a framework, we should consider exposing the plugin system as ContT callbacks to allow our users more power over their control flow. Sometimes the customer just really wants the spaghetti.

For example, if the Scala compiler was written using CPS, it would allow for a principled approach to communication between compiler phases. A compiler plugin would be able to perform some action based on the inferred type of an expression, computed at a later stage in the compile. Similarly, continuations would be a good API for an extensible build tool or text editor.

A caveat with ContT is that it is not stack safe, so cannot be used for programs that run forever. Great, kid. Don’t get ContT.

A more complex variant of ContT called IndexedContT wraps (A => F[B]) => F[C]. The new type parameter C allows the return type of the entire computation to be different to the return type between each component. But if B is not equal to C then there is no Monad.

Not missing an opportunity to generalise as much as possible, IndexedContT is actually implemented in terms of an even more general structure (note the extra s before the T)

  final case class IndexedContsT[W[_], F[_], C, B, A](_run: W[A => F[B]] => F[C])
  type IndexedContT[f[_], c, b, a] = IndexedContsT[Id, f, c, b, a]
  type ContT[f[_], b, a]           = IndexedContsT[Id, f, b, b, a]
  type ContsT[w[_], f[_], b, a]    = IndexedContsT[w, f, b, b, a]
  type Cont[b, a]                  = IndexedContsT[Id, Id, b, b, a]

where W[_] has a Comonad, and ContT is actually implemented as a type alias. Companion objects exist for these type aliases with convenient constructors.

Admittedly, five type parameters is perhaps a generalisation too far. But then again, over-generalisation is consistent with the sensibilities of continuations.

7.4.11 Transformer Stacks and Ambiguous Implicits

This concludes our tour of the monad transformers in scalaz.

When multiple transformers are combined, we call this a transformer stack and although it is verbose, it is possible to read off the features by reading the transformers. For example if we construct an F[_] context which is a set of composed transformers, such as

  type Ctx[A] = StateT[EitherT[IO, E, ?], S, A]

we know that we are adding error handling with error type E (there is a MonadError[Ctx, E]) and we are managing state A (there is a MonadState[Ctx, S]).

But there are unfortunately practical drawbacks to using monad transformers and their companion Monad typeclasses:

  1. Multiple implicit Monad parameters mean that the compiler cannot find the correct syntax to use for the context.
  2. Monads do not compose in the general case, which means that the order of nesting of the transformers is important.
  3. All the interpreters must be lifted into the common context. For example, we might have an implementation of some algebra that uses for IO and now we need to wrap it with StateT and EitherT even though they are unused inside the interpreter.
  4. There is a performance cost associated to each layer. And some monad transformers are worse than others. StateT is particularly bad but even EitherT can cause memory allocation problems for high throughput applications.

Let’s talk about workarounds. No Syntax

Let’s say we have an algebra

  trait Lookup[F[_]] {
    def look: F[Int]

and some data types

  final case class Problem(bad: Int)
  final case class Table(last: Int)

that we want to use in our business logic

  def foo[F[_]](L: Lookup[F])(
      E: MonadError[F, Problem],
      S: MonadState[F, Table]
  ): F[Int] = for {
    old <- S.get
    i   <- L.look
    _   <- if (i === old.last) E.raiseError(Problem(i))
           else ().pure[F]
  } yield i

The first problem we encounter is that this fails to compile

  [error] value flatMap is not a member of type parameter F[Table]
  [error]     old <- S.get
  [error]              ^

There are some tactical solutions to this problem. The most obvious is to make all the parameters explicit

  def foo1[F[_]: Monad](
    L: Lookup[F],
    E: MonadError[F, Problem],
    S: MonadState[F, Table]
  ): F[Int] = ...

and require only Monad to be passed implicitly via context bounds. However, this means that we must manually wire up the MonadError and MonadState when calling foo1 and when calling out to another method that requires an implicit.

A second solution is to leave the parameters implicit and use name shadowing to make all but one of the parameters explicit. This allows upstream to use implicit resolution when calling us but we still need to pass parameters explicitly if we call out.

  @inline final def shadow[A, B, C](a: A, b: B)(f: (A, B) => C): C = f(a, b)
  def foo2a[F[_]: Monad](L: Lookup[F])(
    E: MonadError[F, Problem],
    S: MonadState[F, Table]
  ): F[Int] = shadow(E, S) { (E, S) => ...

or we could shadow just one Monad, leaving the other one to provide our syntax and to be available for when we call out to other methods

  @inline final def shadow[A, B](a: A)(f: A => B): B = f(a)
  def foo2b[F[_]](L: Lookup[F])(
    E: MonadError[F, Problem],
    S: MonadState[F, Table]
  ): F[Int] = shadow(E) { E => ...

A third option, with a higher up-front cost, is to create a custom Monad typeclass that holds implicit references to the two Monad classes that we care about

  trait MonadErrorState[F[_], E, S] {
    implicit def E: MonadError[F, E]
    implicit def S: MonadState[F, S]

and a derivation of the typeclass given a MonadError and MonadState

  object MonadErrorState {
    implicit def create[F[_], E, S](
        E0: MonadError[F, E],
        S0: MonadState[F, S]
    ) = new MonadErrorState[F, E, S] {
      def E: MonadError[F, E] = E0
      def S: MonadState[F, S] = S0

Now if we want access to S or E we get them via F.S or F.E

  def foo3a[F[_]: Monad](L: Lookup[F])(
    implicit F: MonadErrorState[F, Problem, Table]
  ): F[Int] =
    for {
      old <- F.S.get
      i   <- L.look
      _ <- if (i === old.last) F.E.raiseError(Problem(i))
      else ().pure[F]
    } yield i

Like the second solution, we can choose one of the Monad instances to be implicit within the block, achieved by importing it

  def foo3b[F[_]](L: Lookup[F])(
    implicit F: MonadErrorState[F, Problem, Table]
  ): F[Int] = {
    import F.E
  } Composing Transformers

An EitherT[StateT[...], ...] has a MonadError but does not have a MonadState, whereas StateT[EitherT[...], ...] can provide both.

The workaround is to study the implicit derivations on the companion of the transformers and to make sure that the outer most transformer provides everything we need.

A rule of thumb is that more complex transformers go on the outside, with this chapter presenting transformers in increasing order of complex. Lifting Interpreters

Continuing the same example, let’s say our Lookup algebra has an IO interpreter

  object LookupRandom extends Lookup[IO] {
    def look: IO[Int] = IO { util.Random.nextInt }

but we want our context to be

  type Ctx[A] = StateT[EitherT[IO, Problem, ?], Table, A]

to give us a MonadError and a MonadState. This means we need to wrap LookupRandom to operate over Ctx.

Firstly, we want to make use of the .liftM syntax on Monad, which uses MonadTrans to lift from our starting F[A] into G[F, A]

  final class MonadOps[F[_]: Monad, A](fa: F[A]) {
    def liftM[G[_[_], _]: MonadTrans]: G[F, A] = ...

It is important to realise that the type parameters to .liftM have two type holes, one of shape _[_] and another of shape _. If we create type aliases of this shape

  type Ctx0[F[_], A] = StateT[EitherT[F, Problem, ?], Table, A]
  type Ctx1[F[_], A] = EitherT[F, Problem, A]
  type Ctx2[F[_], A] = StateT[F, Table, A]

We can abstract over MonadTrans to lift a Lookup[F] to any Lookup[G[F, ?]] where G is a Monad Transformer:

  def liftM[F[_]: Monad, G[_[_], _]: MonadTrans](f: Lookup[F]) =
    new Lookup[G[F, ?]] {
      def look: G[F, Int] = f.look.liftM[G]

Allowing us to wrap once for EitherT, and then again for StateT

  val wrap1 = Lookup.liftM[IO, Ctx1](LookupRandom)
  val wrap2: Lookup[Ctx] = Lookup.liftM[EitherT[IO, Problem, ?], Ctx2](wrap1)

Another way to achieve this, in a single step, is to use scalaz.effect.LiftIO which enables lifting an IO into a transformer stack:

  @typeclass trait LiftIO[F[_]] {
    def liftIO[A](ioa: IO[A]): F[A]
  @typeclass trait MonadIO[F[_]] extends LiftIO[F] with Monad[F]

with MonadIO instances for all the common combinations of transformers.

The boilerplate overhead to lift an IO interpreter to anything with a MonadIO instance is therefore two lines of code (for the interpreter definition), plus one line per element of the algebra, and a final line to call it:

  def liftIO[F[_]: MonadIO](io: Lookup[IO]) = new Lookup[F] {
    def look: F[Int] = io.look.liftIO[F]
  val L: Lookup[Ctx] = Lookup.liftIO(LookupRandom) Performance

The biggest problem with Monad Transformers is their performance overhead. EitherT has a reasonably low overhead, with every .flatMap call generating a handful of objects, but this can impact high throughput applications where every object allocation matters. Other transformers, such as StateT, effectively add a trampoline, and ContT keeps the entire call-chain retained in memory.

If performance becomes a problem, the solution is to not use Monad Transformers. At least not the transformer data structures. A big advantage of the Monad typeclasses, like MonadState is that we can create an optimised F[_] for our application that provides the typeclasses naturally. We will learn how to create an optimal F[_] over the next two chapters, when we deep dive into two structures which we have already seen: Free and IO.

7.5 A Free Lunch

Our industry craves safe high-level languages, trading developer efficiency and reliability for reduced runtime performance.

The Just In Time (JIT) compiler on the JVM performs so well that simple functions can have comparable performance to their C or C++ equivalents, ignoring the cost of garbage collection. However, the JIT only performs low level optimisations: branch prediction, inlining methods, unrolling loops, and so on.

The JIT does not perform optimisations of our business logic, for example batching network calls or parallelising independent tasks. The developer is responsible for writing the business logic and optimisations at the same time, reducing readability and making it harder to maintain. It would be good if optimisation was a tangential concern.

If instead, we have a data structure that describes our business logic in terms of high level concepts, not machine instructions, we can perform high level optimisation. Data structures of this nature are typically called Free structures and can be generated for free for the members of the algebraic interfaces of our program. For example, a Free Applicative can be generated that allows us to batch or de-duplicate expensive network I/O.

In this section we will learn how to create free structures, and how they can be used.

7.5.1 Free (Monad)

Fundamentally, a monad describes a sequential program where every step depends on the previous one. We are therefore limited to modifications that only know about things that we’ve already run and the next thing we are going to run.

As a refresher, Free is the data structure representation of a Monad and is defined by three members

  sealed abstract class Free[S[_], A] {
    def mapSuspension[T[_]](f: S ~> T): Free[T, A] = ...
    def flatMapSuspension[T[_]](f: S ~> Free[T, ?]): Free[T, A] = ...
    def foldMap[M[_]: Monad](f: S ~> M): M[A] = ...
  object Free {
    private final case class Suspend[S[_], A](a: S[A]) extends Free[S, A]
    private final case class Return[S[_], A](a: A)     extends Free[S, A]
    private final case class Gosub[S[_], A0, B](
      a: Free[S, A0],
      f: A0 => Free[S, B]
    ) extends Free[S, B] { type A = A0 }
    def liftF[S[_], A](value: S[A]): Free[S, A] = Suspend(value)
  • Suspend represents a program that has not yet been interpreted
  • Return is .pure
  • Gosub is .bind

A Free[S, A] can be freely generated for any algebra S. To make this explicit, consider our application’s Machines algebra

  trait Machines[F[_]] {
    def getTime: F[Instant]
    def getManaged: F[NonEmptyList[MachineNode]]
    def getAlive: F[Map[MachineNode, Instant]]
    def start(node: MachineNode): F[Unit]
    def stop(node: MachineNode): F[Unit]

We define a freely generated Free for Machines by creating a GADT with a data type for each element of the algebra. Each data type has the same input parameters as its corresponding element, is parameterised over the return type, and has the same name:

  object Machines {
    sealed abstract class Ast[A]
    final case class GetTime()                extends Ast[Instant]
    final case class GetManaged()             extends Ast[NonEmptyList[MachineNode]]
    final case class GetAlive()               extends Ast[Map[MachineNode, Instant]]
    final case class Start(node: MachineNode) extends Ast[Unit]
    final case class Stop(node: MachineNode)  extends Ast[Unit]

The GADT defines an Abstract Syntax Tree (AST) because each member is representing a computation in a program.

We then define .liftF, an implementation of Machines, with Free[Ast, ?] as the context. Every method simply delegates to Free.liftT to create a Suspend

    def liftF = new Machines[Free[Ast, ?]] {
      def getTime = Free.liftF(GetTime())
      def getManaged = Free.liftF(GetManaged())
      def getAlive = Free.liftF(GetAlive())
      def start(node: MachineNode) = Free.liftF(Start(node))
      def stop(node: MachineNode) = Free.liftF(Stop(node))

When we construct our program, parameterised over a Free, we run it by providing an interpreter (a natural transformation Ast ~> M) to the .foldMap method. For example, if we could provide an interpreter that maps to IO we can construct an IO[Unit] program via the free AST

  def program[F[_]: Monad](M: Machines[F]): F[Unit] = ...
  val interpreter: Machines.Ast ~> IO = ...
  val app: IO[Unit] = program[Free[Machines.Ast, ?]](Machines.liftF)

For completeness, an interpreter that delegates to a direct implementation is easy to write. This might be useful if the rest of the application is using Free as the context and we already have an IO implementation that we want to use:

  def interpreter[F[_]](f: Machines[F]): Ast ~> F = λ[Ast ~> F] {
    case GetTime()    => f.getTime
    case GetManaged() => f.getManaged
    case GetAlive()   => f.getAlive
    case Start(node)  => f.start(node)
    case Stop(node)   => f.stop(node)

But our business logic needs more than just Machines, we also need access to the Drone algebra, recall defined as

  trait Drone[F[_]] {
    def getBacklog: F[Int]
    def getAgents: F[Int]
  object Drone {
    sealed abstract class Ast[A]
    def liftF = ...
    def interpreter = ...

What we want is for our AST to be a combination of the Machines and Drone ASTs. We studied Coproduct in Chapter 6, a higher kinded disjunction:

  final case class Coproduct[F[_], G[_], A](run: F[A] \/ G[A])

Now we can use the context Free[Coproduct[Machines.Ast, Drone.Ast, ?], ?].

We could manually create the coproduct but we would be swimming in boilerplate, and we’d have to do it all again if we wanted to add a third algebra.

The scalaz.Inject typeclass helps:

  type :<:[F[_], G[_]] = Inject[F, G]
  sealed abstract class Inject[F[_], G[_]] {
    def inj[A](fa: F[A]): G[A]
    def prj[A](ga: G[A]): Option[F[A]]
  object Inject {
    implicit def left[F[_], G[_]]: F :<: Coproduct[F, G, ?]] = ...

The implicit derivations generate Inject instances when we need them, letting us rewrite our liftF to work for any combination of ASTs:

  def liftF[F[_]](implicit I: Ast :<: F) = new Machines[Free[F, ?]] {
    def getTime                  = Free.liftF(I.inj(GetTime()))
    def getManaged               = Free.liftF(I.inj(GetManaged()))
    def getAlive                 = Free.liftF(I.inj(GetAlive()))
    def start(node: MachineNode) = Free.liftF(I.inj(Start(node)))
    def stop(node: MachineNode)  = Free.liftF(I.inj(Stop(node)))

It is nice that F :<: G reads as if our Ast is a member of the complete F instruction set: this syntax is intentional.

Putting it all together, lets say we have a program that we wrote abstracting over Monad

  def program[F[_]: Monad](M: Machines[F], D: Drone[F]): F[Unit] = ...

and we have some existing implementations of Machines and Drone, we can create interpreters from them:

  val MachinesIO: Machines[IO] = ...
  val DroneIO: Drone[IO]       = ...
  val M: Machines.Ast ~> IO = Machines.interpreter(MachinesIO)
  val D: Drone.Ast ~> IO    = Drone.interpreter(DroneIO)

and combine them into the larger instruction set using a convenience method from the NaturalTransformation companion

  object NaturalTransformation {
    def or[F[_], G[_], H[_]](fg: F ~> G, hg: H ~> G): Coproduct[F, H, ?] ~> G = ...
  type Ast[a] = Coproduct[Machines.Ast, Drone.Ast, a]
  val interpreter: Ast ~> IO = NaturalTransformation.or(M, D)

Then use it to produce an IO

  val app: IO[Unit] = program[Free[Ast, ?]](Machines.liftF, Drone.liftF)

But we’ve gone in circles! We could have used IO as the context for our program in the first place and avoided Free. So why did we put ourselves through all this pain? Let’s see some reasons why Free might be useful. Testing: Mocks and Stubs

It might sound hypocritical to propose that Free can be used to reduce boilerplate, given how much code we have written. However, there is a tipping point where the Ast pays for itself when we have many tests that require stub implementations.

If the .Ast and .liftF is defined for an algebra, we can create partial interpreters

  val M: Machines.Ast ~> Id = stub[Map[MachineNode, Instant]] {
    case Machines.GetAlive() => Map.empty
  val D: Drone.Ast ~> Id = stub[Int] {
    case Drone.GetBacklog() => 1

which can be used to test our program

  program[Free[Ast, ?]](Machines.liftF, Drone.liftF)
    .foldMap(or(M, D))

By using partial functions, and not total functions, we are exposing ourselves to runtime errors. Many teams are happy to accept this risk in their unit tests since the test would fail if there is a programmer error.

Arguably we could also achieve the same thing with implementations of our algebras that implement every method with ???, overriding what we need on a case by case basis. Monitoring

It is typical for server applications to be monitored by runtime agents that manipulate bytecode to insert profilers and extract various kinds of usage or performance information.

If our application’s context is Free, we do not need to resort to bytecode manipulation, we can instead implement a side-effecting monitor as an interpreter that we have complete control over.

For example, consider using this Ast ~> Ast “agent”

  val Monitor = λ[Demo.Ast ~> Demo.Ast]( match {
      case \/-(m @ Drone.GetBacklog()) =>
      case other =>

which records method invocations: we would use a vendor-specific routine in real code. We could also watch for specific messages of interest and log them as a debugging aid.

We can attach Monitor to our production Free application with


or combine the natural transformations and run with a single

  .foldMap(Monitor.andThen(interpreter)) Monkey Patching: Part 1

As engineers, we know that our business users often ask for bizarre workarounds to be added to the core logic of the application. We might want to codify such corner cases as exceptions to the rule and handle them tangentially to our core logic.

For example, suppose we get a memo from accounting telling us

URGENT: Bob is using node #c0ffee to run the year end. DO NOT STOP THIS MACHINE!1!

There is no possibility to discuss why Bob shouldn’t be using our machines for his super-important accounts, so we have to hack our business logic and put out a release to production as soon as possible.

.flatMapSuspension to the rescue, which is like .mapSuspension but we can also return a pre-canned result (Free.pure) instead of scheduling the instruction. We special case the instruction in a custom natural transformation with its return value:

  val monkey = λ[Machines.Ast ~> Free[Machines.Ast, ?]] {
    case Machines.Stop(MachineNode("#c0ffee")) => Free.pure(())
    case other                                 => Free.liftF(other)

eyeball that it works, push it to prod, and set an alarm for next week to remind us to remove it, and revoke Bob’s access to our servers.

Our unit test could use State as the target context, so we can keep track of all the nodes we stopped:

  type S = Set[MachineNode]
  val M: Machines.Ast ~> State[S, ?] = Mocker.stub[Unit] {
    case Machines.Stop(node) => State.modify[S](_ + node)

along with a test that “normal” nodes are not affected.

An advantage of using Free to avoid stopping the #c0ffee nodes is that we can be sure to catch all the usages instead of having to go through the business logic and look for all usages of .stop. If our application context is just an IO we could, of course, implement this logic in the Machines[IO] implementation but an advantage of using Free is that we don’t need to touch the existing code and can instead isolate and test this (temporary) behaviour, without being tied to the IO implementations. Monkey Patching: Part 2

Infrastructure sends a memo:

To meet the CEO’s vision for this quarter, we are on a cost rationalisation and reorientation initiative.

Therefore, we paid Google a million dollars to develop a Batch API so we can start nodes more cost effectively.

PS: Your bonus depends on using the new API.

When we monkey patch, we are not limited to the original instruction set: we can introduce new ASTs. Rather than change our core business logic, we might decide to translate existing instructions into an extended set, introducing Batch:

  trait Batch[F[_]] {
    def start(nodes: NonEmptyList[MachineNode]): F[Unit]
  object Batch {
    sealed abstract class Ast[A]
    def liftF = ...

Let’s first set up a test for a simple program by defining the AST and target type:

  type Orig[a] = Coproduct[Machines.Ast, Drone.Ast, a]
  type T[a]    = State[S, a]

We track the started nodes in a data container so we can assert on them later

  final case class S(
    singles: IList[MachineNode],
    batches: IList[NonEmptyList[MachineNode]]
  ) {
    def addSingle(node: MachineNode) = S(node :: singles, batches)
    def addBatch(nodes: NonEmptyList[MachineNode]) = S(singles, nodes :: batches)

and introduce some stub implementations

  val M: Machines.Ast ~> T = Mocker.stub[Unit] {
    case Machines.Start(node) => State.modify[S](_.addSingle(node))
  val D: Drone.Ast ~> T = Mocker.stub[Int] {
    case Drone.GetBacklog() => 2.pure[T]

We can expect that the following simple program will behave as expected and call Machines.Start twice:

  def program[F[_]: Monad](M: Machines[F], D: Drone[F]): F[Unit] =
    for {
      todo <- D.getBacklog
      _    <- (1 |-> todo).traverse(id => M.start(MachineNode(id.shows)))
    } yield ()
  program(Machines.liftF[Orig], Drone.liftF[Orig])
    .foldMap(or(M, D))
    .run(S(IList.empty, IList.empty))
    .shouldBe(S(IList(MachineNode("2"), MachineNode("1")), IList.empty))

But we don’t want to use Machines.Start, we need Batch.Start to get our bonus. Expand the AST to keep track of the Waiting nodes that we are delaying, and add the Batch instructions:

  type Waiting      = IList[MachineNode]
  type Extension[a] = Coproduct[Batch.Ast, Orig, a]
  type Patched[a]   = StateT[Free[Extension, ?], Waiting, a]

along with a stub for the Batch algebra

  val B: Batch.Ast ~> T = Mocker.stub[Unit] {
    case Batch.Start(nodes) => State.modify[S](_.addBatch(nodes))

We can convert from the Orig AST into Patched by providing a natural transformation that batches node starts:

  def monkey(max: Int) = new (Orig ~> Patched) {
    def apply[α](fa: Orig[α]): Patched[α] = match {
      case -\/(Machines.Start(node)) =>
        StateT { waiting =>
          if (waiting.length >= max) {
            val start = Batch.Start(NonEmptyList.nel(node, waiting))
              .liftF[Extension, Unit](leftc(start))
          } else
              .pure[Extension, Unit](())
              .strengthL(node :: waiting)
      case _ =>
          .liftF[Extension, α](rightc(fa))
          .liftM[StateT[?[_], Waiting, ?]]

We’re using .strengthL to set the value of the Waiting state, with .pure again letting us avoid sending an instruction in this code branch.

We .foldMap twice because of the state, and combine the stubs again with .or:

  program(Machines.liftF[Orig], Drone.liftF[Orig])
    .run(IList.empty) // starting Waiting list
    .foldMap(or(B, or(M, D)))

Then we run the program and assert: that there are no nodes in the Waiting list, no node has been launched using the old API, and all nodes have been launched in one call to the batch API.

  .run(S(IList.empty, IList.empty))
        IList.empty, // no singles
        IList(NonEmptyList(MachineNode("2"), MachineNode("1"))) // bonus time!
        IList.empty, // no Waiting
        () // the program output

Congratulations, we’ve saved the company $50 every month, and it only cost a million dollars. But that was some other team’s budget, so it is OK.

We could have done the same monkey patch by hard coding the batching logic into our algebra implementations. In the defence of Free, we have decoupled the patch from the implementation, which means we can test it more thoroughly.

7.5.2 FreeAp

Despite this chapter being called Advanced Monads, the takeaway is: don’t use monads unless you really really have to. In this section, we will see why FreeAp (free applicative) is preferable to Free monads.

In the previous Free example, we used the power of the dark side to batch sequential operations. When our context is Applicative, grouping of work is both easier and mathematically correct.

FreeAp is defined as the data structure representation of the ap and pure methods from the Applicative typeclass:

  sealed abstract class FreeAp[S[_], A] {
    def hoist[G[_]](f: S ~> G): FreeAp[G,A] = ...
    def foldMap[G[_]: Applicative](f: S ~> G): G[A] = ...
    def monadic: Free[S, A] = ...
    def analyze[M: Monoid](f: S ~> λ[α => M]): M = ...
  object FreeAp {
    private final case class Pure[S[_], A](a: A) extends FreeAp[S, A]
    private final case class Ap[S[_], A, B](
      value: () => S[B],
      function: () => FreeAp[S, B => A]
    ) extends FreeAp[S, A]
    def apply[S[_], A, B](v: =>F[A], f: =>FreeAp[S, A => B]): FreeAp[S, B] = ...
    def pure[S[_], A](a: A): FreeAp[S, A] = Pure(a)
    def lift[S[_], A](x: => S[A]): FreeAp[S, A] = apply(x, Pure((a: A) => a))

The ADT specific methods .hoist and .foldMap are like their Free analogues .mapSuspension and .foldMap.

As a convenience, we can generate a Free[S, A] from our FreeAp[S, A] with .monadic. This is especially useful to optimise smaller Applicative subsystems yet use them as part of a larger Free program.

.analyze is a TODO (reference previous Const example)

TODO: Optimise network lookup.

TODO: include diagram about cache hits vs network lookups

7.5.3 TODO Coyoneda (FreeFun)

TODO: can we do map fusion?

Functional Programming lends itself well to compiletime optimisations, an area that has not been explored to its full potential. Consider mapping over a Functor three times:

A technique known as map fusion allows us to rewrite this expression as => c(b(a(x)))), avoiding intermediate representations. For example, if xs is a List of a thousand elements, we save two thousand object allocations.

The better-monadic-for project is attempting to implement these middle-level optimisations, which is beyond the scope of this book.

  • Programs that change values
  • Programs that build data
  • Programs that build programs

7.5.4 TODO Free anything

8. The Infinite Sadness

You’ve reached the end of this Early Access book. Please check the website regularly for updates.

You can expect to see chapters covering the following topics:

  • Advanced Monads (more to come)
  • Typeclass Derivation
  • Optics
  • Appendix: Scalaz source code layout

while continuing to build out the example application.

Typeclass Cheatsheet

Typeclass Method From Given To
InvariantFunctor xmap F[A] A => B, B => A F[B]
Contravariant contramap F[A] B => A F[B]
Functor map F[A] A => B F[B]
Apply ap / <*> F[A] F[A => B] F[B]
  apply2 F[A], F[B] (A, B) => C F[C]
Divide divide2 F[A], F[B] C => (A, B) F[C]
Bind bind / >>= F[A] A => F[B] F[B]
  join F[F[A]]   F[A]
Cobind cobind F[A] F[A] => B F[B]
  cojoin F[A]   F[F[A]]
Applicative point A   F[A]
Comonad copoint F[A]   A
Semigroup append A, A   A
Plus plus / <+> F[A], F[A]   F[A]
MonadPlus withFilter F[A] A => Boolean F[A]
Align align F[A], F[B]   F[A \&/ B]
  merge F[A], F[A]   F[A]
Zip zip F[A], F[B]   F[(A, B)]
Unzip unzip F[(A, B)]   (F[A], F[B])
Cozip cozip F[A \/ B]   F[A] \/ F[B]
Foldable foldMap F[A] A => B B
  foldMapM F[A] A => G[B] G[B]
Traverse traverse F[A] A => G[B] G[F[B]]
  sequence F[G[A]]   G[F[A]]
Equal equal / === A, A   Boolean
Show shows A   String
Bifunctor bimap F[A, B] A => C, B => D F[C, D]
  leftMap F[A, B] A => C F[C, B]
  rightMap F[A, B] B => C F[A, C]
Bifoldable bifoldMap F[A, B] A => C, B => C C
(with MonadPlus) separate F[G[A, B]]   (F[A], F[B])
Bitraverse bitraverse F[A, B] A => G[C], B => G[D] G[F[C, D]]
  bisequence F[G[A], G[B]]   G[F[A, B]]

Third Party Licenses

Some of the source code in this book has been copied from free / libre software projects. The license of those projects require that the following texts are distributed with the source that is presented in this book.

Scala License

  Copyright (c) 2002-2017 EPFL
  Copyright (c) 2011-2017 Lightbend, Inc.
  All rights reserved.
  Redistribution and use in source and binary forms, with or without modification,
  are permitted provided that the following conditions are met:
    * Redistributions of source code must retain the above copyright notice,
      this list of conditions and the following disclaimer.
    * Redistributions in binary form must reproduce the above copyright notice,
      this list of conditions and the following disclaimer in the documentation
      and/or other materials provided with the distribution.
    * Neither the name of the EPFL nor the names of its contributors
      may be used to endorse or promote products derived from this software
      without specific prior written permission.

Scalaz License

  Copyright (c) 2009-2014 Tony Morris, Runar Bjarnason, Tom Adams,
                          Kristian Domagala, Brad Clow, Ricky Clarkson,
                          Paul Chiusano, Trygve Laugstøl, Nick Partridge,
                          Jason Zaugg
  All rights reserved.
  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions
  are met:
  1. Redistributions of source code must retain the above copyright
     notice, this list of conditions and the following disclaimer.
  2. Redistributions in binary form must reproduce the above copyright
     notice, this list of conditions and the following disclaimer in the
     documentation and/or other materials provided with the distribution.
  3. Neither the name of the copyright holder nor the names of
     its contributors may be used to endorse or promote products derived from
     this software without specific prior written permission.

spray-json License

spray-json is released under the Apache 2.0 and the following NOTICE

  Copyright (C) 2009-2011 Mathias Doenitz
  Inspired by a similar implementation by Nathan Hamblen
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
  You may obtain a copy of the License at
  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  See the License for the specific language governing permissions and
  limitations under the License.