Skip to main content

Built and signed on GitHub Actions

The fun in functional programming. A collection of algebraic data types, optics, and data structures based on a light weight higher kinded type implementation. Written for deno. Monad, functor, yoneda, yadda yadda yadda.

This package works with Bun, Cloudflare Workers, Node.js, Deno, Browsers
JSR Score
4 weeks ago (2.2.2)

functional Coverage Status deno module JSR

Functional is a no dependency utility library for TypeScript and JavaScript that includes a full suite of functional programming tools. Recent versions have moved away from the jargon of Category Theory to a more intuitive naming scheme unique to functional. However, for those that do have existing experience with functional programming or category theory, functional includes many algebraic data types (including implementations of algebraic structures for native javascript structures), type classes, TypeScript-based higher kinded type substitutions, data-last utility combinators, and concrete type utilities.

The primary goals of functional are to be:

  • Pragmatic: The API surface of functional should favor ease-of-use and consistency over cleverness or purity. This project is ultimately for getting work done, even it if means a data structure or tools is not mathematically sound in all use cases.
  • Understandable: The higher kinded type and algebraic structure implementations are meant to be as simple as possible so their logic can be easily audited. We have also chosen descriptive names for TypeClass implementations instead of ones pulled from Category Theory.
  • Performant: Once the first two goals are satisfied, the long term changes within functional are likely going to be beneath the API surface and aimed at speeding things up where possible.

Some non-goals of functional are:

  • To be an exact port of fp-ts. Many changes have been implemented throughout functional that diverge sharply from fp-ts, this is often on purpose.


This library is a collection of smaller tools. This means that each one should be imported separately. There is no barrel export, instead one should pull in the modules they need individually. Following is an example importing from jsr using the deno runtime.

import * as A from "jsr:@baetheus/fun/array";
import { pipe } from "jsr:@baetheus/fun/fn";

  A.range(5), // get 5 numbers 0-4 => n * n), // square them
  console.log, // [ 0, 1, 4, 9, 16 ]


Documentation is generated for each github tagged release. The latest documentation can be found here. Following is a list of the algebraic data types and algebraic structures/type classes that are implemented in fun. Note that some of these types are both data structures and more general algebraic structures.

Type Algebraic Data Type Algebraic Structure Native Other Names
Applicable Applicative
Bimappable Bifunctor, Covariant Bifunctor
Combinable Semigroup
Comparable Setoid, Eq
Composable Category
Failable Validation
Flatmappable Monad
Foldable Reducible
Initializable Monoid
Mappable Functor, Covariant Functor
Premappable Contravariant, Contravariant Functor
Showable Show
Sortable Ord
Wrappable Pointed
Newtype Brand, Branded Type
ReadonlyArray Array
ReadonlyMap Map
ReadonlySet Set
Async Task
AsyncEither TaskEither
Fn Reader
FnEither ReaderEither
Identity Trivial
Optic Iso, Lens, Optional, Prism, Traversal
Option Maybe
Pair Separated
Stream Observable
Sync IO
SyncEither IOEither

Major Versions

In the fashion of semantic versioning function makes an effort to not break APIs on minor or patch releases. Occasionally, candidate tags (eg. 2.0.0-alpha.1) will be used to indicate a commit is ready for inspection by other developers of fun. The main branch of fun is for bleeding edge developement and is not considered to be a production ready import.

Version Deno Release TypeScript Version
2.0.0 1.36.0 5.1.6
1.0.0 1.9.2 4.2.2


functional started as an exploratory project in late 2020 to learn more about higher kinded type implementations in TypeScript and to assess how much effort it would take to port fp-ts to a Deno-native format. Through that process it became clear that the things I had learned could serve as both useful tools and as a learning resource in and of itself. At various times functional has used multiple hkt encodings, type class definitions, and implementation methods. Some of the key history moments of functional are in the hkts history. Specifically, the hkts implementation in the initial commit and the last major type system rewrite might be interesting. Now, however, the API for version 1.0.0 is set and will only change between major versions (which should be extremely rare).

This project is incredibly indebted to gcanti, pelotom, and the TypeScript community at large. There is nothing new in this project, it's all a reimaginings of ideas that already existed.

For anyone getting started with functional programming I highly recommend writing your own implementation of an ADT such as Option or Either. From Functor to IndexedTraversable with everything inbetween, there is a lot to learn about the mechanics of programming in general by taking these small pieces apart and putting them back together.


Contributions are welcome! Currently, the only maintainer for functional is baetheus. If you want to add to functional or change something, open an issue and ask away. The guidelines for contribution are:

  1. We are kind to others
  2. We use conventional commit messages
  3. We use semantic versioning
  4. We try to keep test coverage at 100%
  5. We try not to break APIs between major releases

Since there is little churn in this library releases are infrequent. If you wish to contribute I would prefer that you start with documentation. It is one of my long term goals to have a few sentences of description and an example for every export. After that, if I'm behind on test coverage that is a great place to start. Last, if you wish to add a feature it's good to start a discussion about the feature with a few concrete use cases before submitting a PR. This will allow for others to chime in without crowding the issues section.

Thanks for you interest!

Built and signed on
GitHub Actions
View transparency log