ADTs for typescript

Enables succinct ADT declaration.

In typescript, Algebraic Data Types are encoded using union types with a discriminator field.

Example:

import { ADT } from 'ts-adt'

type Option<A> = ADT<{
  some: {value: A},
  none: {}
}>

This translates to:

type Option<A> = { _type: 'some', value: A} | { _type: 'none' }

Here, Option<A> represents a value that can be one of two types; either it's an object with a _type attribute "some" and a value A, or it's an object with _type attribute "none". This type is quite useful, especially when used with Typescript's type narrowing feature:

declare const userImage: Option<string>

function getUserImage(): string {
  if(userImage._type === 'some') {
    return userImage.value // value is accessible here, since _type is 'some'
  } else {
    return "http://example.com/defaultImage" 
  }
}

Pattern matching is a common usecase when dealing with ADTs. You'll often find yourself in a position where you need to default to certain values depending on what the underlying value is. For this reason, we've included a match and matchI function:

import { ADT, match, matchI } from 'ts-adt'

declare const userImage: Option<string>

const img = matchI(userImage)({
  some: ({value}) => value,
  none: () => "http://example.com/defaultImage"
})

const img = pipe(
  userImage,
  match({
    some: ({value}) => value,
    none: () => "http://example.com/defaultImage"
  })
)

Both functions work the same way, but their arguments are ordered differently for better inference in different cases.