So you've learned Haskell. You've taught your friends about monads, you've worked through some beginner textbooks, and maybe you've played around with some open source projects. Now that you've had a taste, you want more: you want to write an application in Haskell for fun! Maybe you want to use Haskell at work!

You sit down at your computer, and you're stuck.

How does anyone actually get anything done with this language?

This is a common thing to wonder.

Haskell has always enjoyed a wide variety of high quality learning material for advanced parts of the language, if you're not afraid of academic papers. The last five years have seen an upswelling of fantastic resources for learning the language as a beginner. However, the language does not have many resources for using it in production. It's difficult to navigate the ecosystems and identify quality resources that are in alignment with your goals and values.

This book aims to help with that situation. After reading this book, you should feel comfortable writing large software projects in Haskell, evaluating competing libraries and techniques, and productively reading material from a variety of Haskell users.

An Opinionated Tour Guide

Haskell is a hugely diverse landscape.

There are many regional groups: United Kingdom, Scandinavia, mainland Europe, Russia, the USA, Japan, China, and India all have thriving Haskell ecosystems that have interesting dialects and differences in custom and practice.

People come to Haskell with many backgrounds. Some people learned Haskell well into their careers, and had a long career writing Java, Scala, or C# beforehand. Some people came to Haskell from dynamically typed languages, like LISP or Ruby. Some people started learning Haskell very early on in their programming career, and use it as the basis of comparison. Some people primarily use Haskell in academic research, while others primarily use Haskell in industrial applications. Some people are hobbyists and just like to write Haskell for fun!

This book is intended for people that want to write Haskell in industry. The tradeoffs and constraints that industrial programmers face are different from academic or hobbyist programmers. This book will cover not only technical aspects of the Haskell language, but also social and engineering concerns that aren't "really" about Haskell.

Part of this book will be objective. I will teach you how to use some interesting techniques and ideas to make developing with Haskell more productive. We'll learn about Template Haskell, type-level programming, and other fun topics.

However, for the most part, this book is inherently subjective. Because Haskell serves so many ecosystems, it is imperative to discern what ecosystem a something is intended for. More than just giving out prescriptions - "This library is production ready! This is a toy!" - I hope to show my thought process and allow you to make your own judgment calls.

Ultimately, this is a book about the social reality of software engineering in niche languages.

About the Author

I'm Matt Parsons. I started learning programming in January 2014 with Java because I wanted to change careers. I learned JavaScript that summer with the excellent book [Eloquent JavaScript][eloquent-javascript]. It had a chapter on functional programming and object oriented programming. I found the chapter on functional programming more intuitive, so I made a mental note to learn the most functional language I could find. A few months later, I started learning Haskell.

My brain picked up Haskell extremely quickly. I had barely started learning imperative and object-oriented programming, so the difficult novelty of learning new jargon and concepts was expected. In August 2015, I started a Haskell internship, and I was able to use Haskell in my Artificial Intelligence coursework. In my last semester of college, I was able to use Haskell in my undergraduate thesis to study the connection between category theory, modal logic, and distributed systems.

I was extremely fortunate to have these opportunities, as they set me up for success to work with Haskell. My first job out of college was converting PHP applications to greenfield Haskell, and I've been working full-time with Haskell ever since. I've worked in a variety of contexts: a startup that wasn't 100% sold on Haskell, a larger company that was sold on Haskell but wrestling with social and technical difficulties of a huge code base and development team, and a startup that was sold on Haskell and working on growing. I also contribute to many open source projects, and I'm familiar with most of the ecosystems. All told, I have worked with nearly a million lines of Haskell code!

I've seen Haskell fail, and I've seen it succeed. I'd like to help you succeed with Haskell.