npm package discovery and stats viewer.

Discover Tips

  • General search

    [free text search, go nuts!]

  • Package details

    pkg:[package-name]

  • User packages

    @[username]

Sponsor

Optimize Toolset

I’ve always been into building performant and accessible sites, but lately I’ve been taking it extremely seriously. So much so that I’ve been building a tool to help me optimize and monitor the sites that I build to make sure that I’m making an attempt to offer the best experience to those who visit them. If you’re into performant, accessible and SEO friendly sites, you might like it too! You can check it out at Optimize Toolset.

About

Hi, 👋, I’m Ryan Hefner  and I built this site for me, and you! The goal of this site was to provide an easy way for me to check the stats on my npm packages, both for prioritizing issues and updates, and to give me a little kick in the pants to keep up on stuff.

As I was building it, I realized that I was actually using the tool to build the tool, and figured I might as well put this out there and hopefully others will find it to be a fast and useful way to search and browse npm packages as I have.

If you’re interested in other things I’m working on, follow me on Twitter or check out the open source projects I’ve been publishing on GitHub.

I am also working on a Twitter bot for this site to tweet the most popular, newest, random packages from npm. Please follow that account now and it will start sending out packages soon–ish.

Open Software & Tools

This site wouldn’t be possible without the immense generosity and tireless efforts from the people who make contributions to the world and share their work via open source initiatives. Thank you 🙏

© 2024 – Pkg Stats / Ryan Hefner

@elmish-ts/tagged-union

v2.0.2

Published

Utilities for defining tagged sum types and pattern matching in TypeScript

Downloads

10

Readme

@elmish-ts/tagged-union

Define tagged unions a little more concisely and get pattern matching for free.

npm type definitions npm (scoped with tag) Coverage Status Build Status npm bundle size (scoped) npm bundle size (scoped)

Install

npm install @elmish-ts/tagged-union

Compatibility

Currently tested against TypeScript v3.5.1. Higher versions should be fine, lower versions may require you to provide more type annotations, or they may not work at all.

Docs

Documentation is built from master and hosted here: https://elmish-ts.github.io/tagged-union/

Description & Usage

Typically defining and working with a tagged sum (a.k.a. "discriminated union") in TypeScript is done by defining unions of record/struct types directly, which requires you to think about some minutiae every time you want to define one:

  • what should the "discriminant" field be called?
  • what other fields should each variant have?
  • does it matter if multiple variants have any field names in common? (yes)

And then when you want to start working with values that inhabit your tagged union, the most straightforward way to do so is to use switch () {} statements, switching on the discriminant field. Switch statements work fine, but you need make sure you take steps to ensure TypeScript enforces exhaustiveness checking so you handle all variants of the union. Also, folks who like functional programming tend to prefer expressions over statements, so it's a little irksome to use switch statements when handling tagged sums.

Here's an example of defining the well know Maybe<A> type using the typical method:

type Maybe<A> =
  | { tag: 'Nothing' }
  | { tag: 'Just', Just: A }

const Nothing: Maybe<never> = {
  tag: 'Nothing'
}
const Just = <A>(a: A): Maybe<A> =>
  ({ tag: 'Just', Just: a })

function map<A>(ma: Maybe<A>, fn: (a: A) => B): Maybe<B> {
  switch (ma.tag) {
    case "Nothing":
      return Nothing

    case 'Just':
      return Just(fn(ma.Just))
  }
}

With this library, we can do the same thing like this:

import { Def, def, caseWhen } from '@elmish-ts/tagged-union'

type Maybe<A> =
  | Def<'Nothing'>
  | Def<'Just', [A]>

const Nothing: Maybe<never> = def('Nothing')
const Just = <A>(a: A): Maybe<A> => def('Just', a)

function map<A>(ma: Maybe<A>, fn: (a: A) => B): Maybe<B> {
  return caseWhen(ma, {
    Nothing: () => Nothing
    Just: a => Just(fn(a))
  })
}

Additionally, you can use an underscore fallback pattern _ if you want handle only some variants explicitly, and handle the rest with a fallback value.

type These<A, B> = Def<'This', [A]> | Def<'That', [B]> | Def<'Both', [A, B]>

const This = <A, B = never>(a: A): These<A, B> => def('This', a)
const That = <A = never, B = unknown>(b: B): These<A, B> => def('That', b)
const Both = <A, B>(a: A, b: B): These<A, B> => def('Both', a, b)

function getBothOr<A, B>(fallback: [A, B]): (these: These<A, B>) => [A, B] {
  return caseOf({
    Both: (a, b): [A, B] => [a, b],
    _: () => fallback
  })
}

Pattern matching is made possible by the functions caseOf and caseWhen. caseOf is curried and takes data last in order to be usable in a point-free fashion in functional pipelines. caseWhen takes the data first and is not curried. Otherwise, they work the same.

Other Examples

Lists

type List<A> = Def<'Nil'> | Def<'Cons', [MoreList<A>]>

interface MoreList<A> {
  head: A
  tail: List<A>
}

const Nil: List<never> = def('Nil')
function Cons<A>(head: A): (tail: List<A>) => List<A> {
  return tail => def('Cons', { head, tail })
}

function map<A, B>(f: (a: A) => B): (ls: List<A>) => List<B> {
  return caseOf({
    Nil: () => Nil,
    Cons: ({ head, tail }) => Cons(f(head), map(f)(tail))
  })
}

Either<L, R>

type Either<L, R> = Def<'Left', [L]> | Def<'Right', [R]>

function Left<L, R = never>(l: L): Either<L, R> {
  return def('Left', l)
}

function Right<L = never, R = unknown>(r: R): Either<L, R> {
  return def('Right', r)
}

function map<L, A, B>(f: (r: A) => B): (e: Either<L, A>): Either<L, B> {
  return caseOf({
    Left: l => Left(l),
    Right: r => Right(f(r))
  })
}