About this book

This is a book on the functional paradigm in general. We'll use the world's most popular functional programming language: JavaScript. Some may feel this is a poor choice as it's against the grain of the current culture which, at the moment, feels predominately imperative. However, I believe it is the best way to learn FP for several reasons:

  • You likely use it every day at work.

    This makes it possible to practice and apply your acquired knowledge each day on real world programs rather than pet projects on nights and weekends in an esoteric FP language.

  • We don't have to learn everything up front to start writing programs.

    In a pure functional language, you cannot log a variable or read a DOM node without using monads. Here we can cheat a little as we learn to purify our codebase. It's also easier to get started in this language since it's mixed paradigm and you can fall back on your current practices while there are gaps in your knowledge.

  • The language is fully capable of writing top notch functional code.

    We have all the features we need to mimic a language like Scala or Haskell with the help of a tiny library or two. Object-oriented programming currently dominates the industry, but it's clearly awkward in JavaScript. It's akin to camping off of a highway or tap dancing in galoshes. We have to bind all over the place lest this change out from under us, we have various work arounds for the quirky behavior when the new keyword is forgotten, private members are only available via closures. To a lot of us, FP feels more natural anyways.

That said, typed functional languages will, without a doubt, be the best place to code in the style presented by this book. JavaScript will be our means of learning a paradigm, where you apply it is up to you. Luckily, the interfaces are mathematical and, as such, ubiquitous. You'll find yourself at home with Swiftz, Scalaz, Haskell, PureScript, and other mathematically inclined environments.

Read it Online

For a best reading experience, read it online via Gitbook.

  • Quick-access side-bar

  • In-browser exercises

  • In-depth examples

Play Around with Code

To make the training efficient and not get too bored while I am telling you another story, make sure to play around with the concepts introduced in this book. Some can be tricky to catch at first and are better understood by getting your hands dirty. All functions and algebraic data-structures presented in the book are gathered in the appendixes. The corresponding code is also available as an npm module:

$ npm i @mostly-adequate/support

Alternatively, exercises of each chapter are runnable and can be completed in your editor! For example, complete the exercise_*.js in exercises/ch04 and then run:

$ npm run ch04

Download it

Find pre-generated PDF and EPUB as build artifacts of the latest release.

Do it yourself

⚠️ This project setup is now a bit old and thus, you may run into various issues when building this locally. We recommend to use node v10.22.1 and the latest version of Calibre if possible.

git clone
cd mostly-adequate-guide/
npm install
npm run setup
npm run generate-pdf
npm run generate-epub

Note! To generate the ebook version you will need to install ebook-convert. Installation instructions.

Table of Contents








Plans for the future

  • Part 1 (chapters 1-7) is a guide to the basics. I'm updating as I find errors since this is the initial draft. Feel free to help!

  • Part 2 (chapters 8-13) address type classes like functors and monads all the way through to traversable. I hope to squeeze in transformers and a pure application.

  • Part 3 (chapters 14+) will start to dance the fine line between practical programming and academic absurdity. We'll look at comonads, f-algebras, free monads, yoneda, and other categorical constructs.

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