Core Concepts

The goal of fp-ts is to empower developers to write pure FP apps and libraries built atop higher order abstractions. It includes the most popular data types, type classes, and abstractions from languages like Haskell, PureScript, and Scala.

Don’t panic. If “higher order abstractions” and “type classes” sound like insanely complicated words to you, the first thing you should know is that you don’t have to understand all the intricate details of fp-ts in order to make good use of it. There is a learning curve, certainly, but don’t fear that you can’t learn to use fp-ts effectively, even as a beginner in functional programming. The trick is to start using the parts that are easier to understand and then gradually expand your knowledge.

There’s a list of recipes to help you find quick solutions to common problems.


Functional programming is all about pure functions and how to compose them into bigger structures. fp-ts provides a few general functions to support you with composition, constant functions, and more.

Data Types

Data types are the practical part of fp-ts: you can instantiate them with your data to gain properties and functionality that are useful for solving a specific need. Because data types all share common interfaces (through type classes), once you learn how to use one data type, you can apply the same concepts to the others.

Many functions in fp-ts use ad hoc polymorphism, meaning that they have a single implementation that can deal with arguments of different types. To make this work, it is often necessary to provide a data type instance that provides functionality that is specific to the data type. Here is an example:

import { option } from 'fp-ts/lib/Option'
import { sequenceS } from 'fp-ts/lib/Apply'

// sequenceS is an ad hoc polymorphic function and requires an Option instance.
// This is what option.option provides.
const sequenceSOption = sequenceS(option)

Here’s a small list of some commonly used data types, you can find more in the API reference.

  • Option – is used to explicitly express that a value can be absent.
  • Either – represents a value of one of two possible types: Left<A> and Right<B>.
  • NonEmptyArray – an array that is guaranteed to contain at least one element.
  • Task – represents an asynchronous computation that yields a value and never fails.

It can be useful to define your own data types, in which case you could use a tool like fp-ts-codegen as a starting point.

Type Classes

Type classes provide the theoretical underpinnings of fp-ts: they describe what you can do with your data. To guarantee that they can be safely composed, they are built on laws rooted in category theory.

A concrete example: A Functor is a type constructor which supports a mapping operation map. Instances must satisfy the following laws:

  1. Identity:, a => a) = fa
  2. Composition:, a => bc(ab(a))) =, ab), bc)

Besides Functor the following type classes are a good starting point:

  • Apply - allows applying a function to an argument under a type constructor.
  • Chain - extends the Apply type class with a chain operation which composes computations in sequence, using the return value of one computation to determine the next computation.
  • Eq - represents types which support decidable equality with the equals operation.

If you’re interested in learning more about this topic, have a look at the learning resources.

Type Class Diagram

Click the image for a larger view with nodes that link to the respective documentation pages.

Type class diagram

Higher Kinded Types

A distinctive feature of fp-ts with respect to other functional libraries is its implementation of Higher Kinded Types, which TypeScript doesn’t support natively. The idea for emulating higher kinded types in TypeScript is based on Lightweight higher-kinded polymorphism.