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Functional programming in TypeScript

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📢 Important Announcement: fp-ts is Joining the Effect-TS Ecosystem!

We are excited to announce that the fp-ts project is officially merging with the Effect-TS ecosystem. This is a significant step forward in the functional programming landscape, bringing together two powerful libraries under one roof. Giulio Canti, the author of fp-ts, is being welcomed into the Effect organization, promising an exciting future with enhanced capabilities and support.

What This Means for New Users:

Effect-TS can be regarded as the successor to fp-ts v2 and embodies what would be considered fp-ts v3. This merger marks a significant evolution in the library's capabilities, integrating more features and functionalities tailored towards robust, type-safe, and scalable functional programming.

For more details on this merger and what it entails, please refer to the official announcement here. Additionally, you can explore more about Effect-TS and its offerings on our website and documentation.


fp-ts is a library for typed functional programming in TypeScript.

fp-ts aims to allow developers to use popular patterns and abstractions that are available in most functional languages. For this, it includes the most popular data types, type classes and abstractions such as Option, Either, IO, Task, Functor, Applicative, Monad to empower users to write pure FP apps and libraries built atop higher order abstractions.

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.

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To install the stable version:

npm install fp-ts

Make sure to always have a single version of fp-ts installed in your project. Multiple versions are known to cause tsc to hang during compilation. You can check the versions currently installed using npm ls fp-ts (make sure there's a single version and all the others are marked as deduped).

TypeScript compatibility

Strictness – This library is conceived, tested and is supposed to be consumed by TypeScript with the strict flag turned on.

fp-ts version required typescript version
2.0.x+ 3.5+
1.15.x+ 3.1+
<= 1.14.4 2.8+ (*)

(*) If you are running < typescript@3.0.1 you have to polyfill the unknown type. You can use unknown-ts as a polyfill.


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


If you need help with fp-ts check out:



The MIT License (MIT)