Lenses for the Mere Mortal: PureScript Edition
Lenses for the Mere Mortal: PureScript Edition
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Lenses for the Mere Mortal: PureScript Edition

Last updated on 2018-08-20

About the Book

Three things about this book make it a novelty among lens tutorials:

  1. Explanations are driven not by lens types, but by the types of data you want to use lenses on. Consider Maps. Most lens tutorials don't cover them. (I'm not sure I've found any that do.) Once you've learned lenses well, it's relatively easy to deduce how to use them with Maps. But if you try to use lenses on Maps before then, you will suffer. Which seems wrong, given how common Maps are in code. So this book covers them in chapter 2.
  2. I wrote the book as I was learning lenses and applying them to my own code. That gives me a better idea of what novices will stumble over (Maps! Error messages!) and allows me to introduce topics in something like a "just in time" order. To the best of my ability, the book represents what I'd have wished my learning experience to be: answers coming just as I was getting puzzled, answers that don't overwhelm me with completeness or detail I'm unprepared for, and so on. This isn't the first time I've written in this style, and it turns out that my ideal learning journey works for other people.
  3. It's a book. That means it can take its time. It doesn't have to leave things for you to figure out from API documentation.

There are other differences, but I'll leave them to the Introduction.

The book uses a PureScript version of lenses because I happened to be learning PureScript when I got the idea of writing it. I hope it's still useful for people wanting to learn the Haskell version. (The two APIs are very similar, though the underlying implementations differ in some important ways.)

I assume only shallow knowledge of PureScript, roughly equivalent to the three post-Elm introductory chapters in An Outsider's Guide to Statically Typed Functional Programming.

About the Author

Brian Marick
Brian Marick

Brian Marick was first exposed to the functional style in 1983, when the accident of knowing a little bit of Lisp tossed him into the job of technical lead on a project to port Common Lisp to a now-defunct computer architecture. That led him to a reading spree about all things Lisp, the language from which the functional style arguably originated. He’s been a language geek ever since, despite making most of his living as a software process consultant. He’s the author of the popular Midje testing library for Clojure and has written books (Everyday Scripting with Ruby, Programming Cocoa with Ruby, and Functional Programming for the Object-Oriented Programmer). The two books in progress are An Outsider's Guide to Statically Typed Functional Programming and Lenses for the Mere Mortal.

Table of Contents

    • Introduction
      • Optics are useful
      • Why are optics notoriously confusing?
      • This book’s approach
      • Why PureScript?
      • Getting ready
      • About the cover
      • Your problems and questions
      • Change log
      • Thanks for the help
    • 1. Tuples and records (lenses for product types)
      • 1.1 Tuples
      • 1.2 Records
      • 1.3 Composing lenses
      • 1.4 Composition exercise
      • 1.5 Types
      • 1.6 Composition exercise two
      • 1.7 Lens laws
      • 1.8 Law exercise
      • 1.9 What now?
    • 2. Maps (At lenses)
      • 2.1 A custom implementation
      • 2.2 At.at
      • 2.3 Types
      • 2.4 Composing At lenses
      • 2.5 What now?
    • 3. Optics and refactoring (optional)
      • 3.1 Architecture
      • 3.2 Optics
      • 3.3 Exercise
      • 3.4 My solution
      • 3.5 What now?
    • 4. Operating on whole collections (Traversal, part 1)
      • 4.1 Working with all the values
      • 4.2 view
      • 4.3 Focusing on individual values
      • 4.4 preview
      • 4.5 Composing traversals
      • 4.6 A sort of base class
      • 4.7 What now?
    • 5. Single elements in arrays and fixed-size maps (Index)
      • 5.1 ix, at, and Map
      • 5.2 Array
      • 5.3 Types
      • 5.4 Composition
      • 5.5 What now?
    • 6. Sum types (prisms)
      • 6.1 Using prisms
      • 6.2 Making a prism
      • 6.3 Prism laws
      • 6.4 Value constructors with more than one argument
      • 6.5 Types
      • 6.6 Composing prisms
      • 6.7 Prisms aren’t just for sum types (exercises)
      • 6.8 What now?
    • 7. Miscellaneous other optics
  • Appendices
    • A cheat sheet for the optic types
      • Lens
      • class At
      • Traversal
      • class Index
      • Prism
    • A catalog of compositions
      • Lens <<< optic
      • Prism <<< optic
      • Traversal <<< optic
      • At lens <<< optic
      • Index optic <<< optic
    • Identifying an optic from type spewage
      • I see Choice
      • I see Strong alone
      • I see Strong and Choice
      • I see Strong and At
      • I see Index and Wander
      • I see Wander alone
  • Notes

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