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Typed functional programming in TypeScript

fp-ts provides developers with popular patterns and reliable abstractions from typed functional languages in TypeScript.

Disclaimer. Teaching functional programming is out of scope of this project, so the documentation assumes you already know what FP is.

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.


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.

Note. Data types are not stack safe and there is no trampolining implementation. But for traversing and sequencing lists there is a stack safe and optimized version in each data types.

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 abstract algebra and category theory.

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.