fp-tx

fp-tx is a continuation of fp-ts, a functional programming library for TypeScript inspired by PureScript and Haskell.

Motivation

Effect has officially replaced fp-ts, and there are many upsides to Effect. For example, there is no need to worry about picking an effect type, there is only one: Effect. It handles synchronous and asynchronous computation, stack-safe recursion, graceful exits, dependency composition, and comes with pre-constructed services and providers.

However, the core philosophy of fp-ts, inspired by typeclasses and monadic composition, makes it a valuable choice. The effect types in fp-ts are dead simple, easy to understand, and their inner mechanisms are transparent to users of the library. The downsides to this approach, namely: stack traces, error handling as a discipline, and stack safety are greatly outweighed by the purely functional nature and design of the library.

@fp-tx was written as a passion project to revive fp-ts and improve on its critical downsides.

Migration

The API surface of @fp-tx/core for versions 0.x and 1.x are intentionally identical to fp-ts. This is so that existing libraries and apps can migrate by simply replacing the import fp-ts(/(lib | es6))/* with @fp-tx/core/*

Comparison to fp-ts

ES Modules

fp-ts has a critical weakness which inspired this entire project: incompatibility with Node ESModules. Within the last few years, certain libraries have adopted an ESM only philosophy which greatly restricted interoperability with other parts of the JavaScript ecosystem, like fp-ts. fp-ts supports two module types: CommonJS and ES6 (a name for tree-shakable module code for browsers and bundlers). However, when Node adopted ESM it took a hard stance for when it would parse module code. If a package.json has "type": "module" it will interpret JS files as containing import / export (ESM). And it will restrict any JS file which contains require, requiring those files to have a .cjs extension. And the opposite is true, if the package.json omits or lists "type": "commonjs" Node will interpret JS files as containing require, and interpret .mjs as containing import and export. The compatibility issue arises when a library forces a Node project to be written in ESM. That project will no longer be compatible with fp-ts.

New Features

@fp-tx/core comes with juicy new features, below are a list of current and future features which aim to improve fp-ts in key areas while maintaining its core set of values.

ChainRec

@fp-tx/core comes with a complete suite of ChainRec instances (see @jacob-alford/chain-rec). ChainRec largely overcomes the stack-safety problem when constructing recursive effects.

do notation

Inspired by effect's gen notation and powered by ChainRec, @fp-tx/core comes with do notation for any data type which supports a ChainRec instance.

import * as E from '@fp-tx/core/Either'

declare const lastLogin_: E.Either<string, Date>

declare const name_: E.Either<Error, string>

//    ↓↓↓↓↓↓↓↓ has type E.Either<string | Date, string>
const greeting = E.do(function* ($) {
  const lastLogin = yield* $(lastLogin_)
  //    ↑↑↑↑↑↑↑↑↑ has type Date
  const name = yield* $(name_)
  //    ↑↑↑↑ has type string
  return `Welcome ${name}!  Your last login was ${lastLogin.toLocaleString()}`
})

Roadmap

0.x and 1.x

@fp-tx/core is starting off with 0.x to ensure that the build and pack processes – which guarantee compatibility with all library types and build processes – is solid. A version starting with 0.x permits breaking changes between minor version updates, this means that if you've installed any 0.x version, the package manager you choose will not update a minor version automatically. Once the build process has been thoroughly tested, 1.0 will be forthcoming.

Additionally, the following modules are planned for release once development of the organization slows down:

• StateReaderTask
• StateTaskEither
• StateTask
• StateEither
• Bifoldable and Bitraversable: a port of @jacob-alford/bifold-traverse

2.0

2.0 will be the coveted 3.0 release of fp-ts which took a few different forms before being cancelled for effect. @fp-tx/core will largely be the same to make transitioning easier, but will properly adopt much needed improvements:

• A new HKT mechanism: TypeLambdas like those in @effect/typeclass, the n-ary HKT system proposal by @skeate, or others!
• Combinator widening by default
• A better and publicly exposed dual system

Contribution and Community

@fp-tx readily welcomes community run libraries with a few caveats. The library must use @fp-tx/build-tools under the hood, and have been constructed using the GitHub template or @fp-tx/create. This guarantees that @fp-tx libraries will be compatible with other JavaScript libraries. These template constructors are not currently available but are planned for the near future.

@fp-tx Organization Roadmap

1. @fp-tx/schema – a port of schemata-ts, a modern feature rich schema validation library
2. @fp-tx/parser – based on parser-ts, a port of purescript-eulalie
3. @fp-tx/trout – bidirectional type-safe route declarations
4. @fp-tx/services – common services such as Fs, HttpRequests, and Logging
5. @fp-tx/providers – platform agnostic service implementations
6. @fp-tx/node – service implementations for Node apps
7. @fp-tx/web – service implementations for the browser
8. @fp-tx/template – a template repository for fp-tx libraries
9. @fp-tx/create – a CLI tool for generating new fp-tx libraries