@unsplash/sum-types

Safe, ergonomic, non-generic sum types in TypeScript.

Documentation: unsplash.github.io/sum-types

import * as Sum from "@unsplash/sum-types"

type Weather = Sum.Member<"Sun"> | Sum.Member<"Rain", number>
const Weather = Sum.create<Weather>()

const getRainfall = Weather.match({
  Rain: n => `${n}mm`,
  Sun: () => "none",
})

const todayWeather = Weather.mk.Rain(5)

getRainfall(todayWeather) // '5mm'

Installation

The library is available on the npm registry: @unsplash/sum-types

Note that due to usage of Proxy and Symbol this library only supports ES2015+.

The following bindings are also available:

• @unsplash/sum-types-fp-ts
• @unsplash/sum-types-io-ts
• @unsplash/sum-types-fast-check

Motivation

The library solves a number of problems we've experienced at Unsplash with alternative libraries in this space. Specifically:

• Convenient member constructor functions are provided, unlike ts-adt.
• The API is small, simple, and boilerplate-free, unlike tagged-ts.
• Pattern matching is curried for use in pipeline application and function composition, unlike @practical-fp/union-types.
• Types are not inlined in compiler output, improving readability and performance at scale, unlike unionize.

The compromise we've made to achieve this is to not support generic sum types.