Folktale by Example


This book is a work in progress!

This is largely a draft at this point, so if you see any problems, feel free to file a ticket and I’ll get to fixing it up asap.

Category Theory is a relatively new branch of mathematics with fairly abstract concepts. Functional programming libraries use such concepts for maximising their abstraction power and general usefulness, but it comes with a certain drawback: most of the constructions provide little or no guidance for concrete use cases. This is a problem in particular with newcomers to this style of programming, who often find themselves lost, asking questions like “But why are Monads useful?”

In this book, you will walk through concrete applications of concepts in Category Theory, Abstract Algebra, and other branches of mathematics used by functional programming, as well as concepts from functional programming itself. By looking at concrete instances of these concepts, you can build a mental framework for generalising and abstracting problems in a way that makes your code more reusable, and more robust.


Do note that this isn’t a book about Category Theory or any other mathematical field, the concepts presented in this book are just influenced by them.


People tend to have a fairly difficult time reasoning about abstractions, but they can easily recognise concrete instances of those abstractions. With enough examples, they can then build their own mental model of that abstraction and, having that mental model, they’ll be able to apply that generalisation to find other concrete instances of that abstractions on their own.

With that in mind, this book tries to present its readers with concrete applications of a concept before discussing the concept itself. It does so by presenting, at each chapter, a set of related problems, discussing concrete solutions for those problems, and finally extrapolating to a general solution that captures the pattern in those concrete solutions.

Who should read this book?

This book is aimed at intermediate and advanced JavaScript programmers who want to take advantage of mathematical concepts to make their JavaScript code bases simpler, more robust and reusable.

You’re expected to be comfortable not only with the syntax and basic concepts of the JavaScript language, but also with concepts such as higher-order programming, first-class functions, objects, prototypes, and dynamic dispatch, which are going to be the basis for the concepts discussed in this book. Non-JavaScript programmers familiar with those concepts might be able to translate the concepts to their languages, with some work, but a different book might be better suited for their needs.

To make the most out of this book, you’ll also need some school-level mathematical reasoning skills, since the generalisation of the concepts will be presented as mathematical laws. Properly understanding them will take some knowledge of equality, substitutability and unification. Albert Y. C. Lai has described the prerequisite mathematical skills for functional programming on a web page.

How is this book organised?

The book is split into a few sections, each section starts with a description its theme, prerequisites and motivation, spends a few chapters talking about concrete examples inside that theme, and concludes with a summary of the abstractions presented. Sections build on top of each other, so it might be difficult to read the book in a non-linear way.

Section 1 discusses functions, and how composition can be used to build new functionality from existing one easily, as well as how JavaScript function affect composition in JS. It presents the core.lambda, core.arity and core.operator libraries.

Section 2 discusses data structures and their transformations. It talks about concepts such as Functors and recursion schemes such as Catamorphisms. This section also gives a general idea of how sum types are modelled in Folktale. It presents the data.maybe, data.either and data.validation libraries.

Section 3 discusses advanced transformations of data structures. It talks about concepts such as Applicative Functors and Monads. It presents new facets of the libraries presented in Section 2.

Section 4 discusses approaches to asynchronous concurrency when dealing with simple values. It revisits Monads, and talks about new concepts such as Continuation-Passing style, Tasks, and Futures. It presents the data.task library.

Section 5 discusses advanced approaches to dealing with multi-value concurrency. It expands on Section 4 by presenting new concepts such as back-pressure, Signals and Channels. It presents the and data.signal libraries.

Section 6 discusses data validation and normalisation in more detail. It presents the data.validation and core.check libraries.


The book contains a heavy amount of examples, all of which can be found in the Folktale GitHub repository, under the docs/examples folder.

Table of Contents