adt-constructors

It is useful in Typescript to have union types distinguished by some particular shared property, called a "discriminant"[^1]. For example, in the following:

type Foo = { tag: 'bar', value: string } | { tag: 'baz' }

The property tag is shared among all possible values of Foo, but its value differs. Given a value of type Foo, you can know whether or not it has a value property by checking if its tag property equals 'bar'.

declare const foo: Foo

if (foo.tag === 'bar') {
  console.log(foo.value) // typescript knows foo.value here is a string
}

This pattern is extremely common, but requires a fair amount of boilerplate to actually implement. There are some libraries (for example, ts-adt) which alleviate some of the pain of actually declaring the types, but you still need to construct the values on your own.

This library exists to help provide a means to construct values of these types.

Usage

This library exports two functions: constructors and makeConstructors. The first is meant for unions discriminated on the key named _type (which is the default of ts-adt). The second allows you to customize this aspect (in fact, constructors is really just makeConstructors('_type')).

If your constructors are simple, then you can easily create them by passing an array of the discriminant key values. This array will offer autocomplete suggestions if your editor supports that, making the process extremely simple.

import type { ADT } from 'ts-adt'
import { constructors } from 'adt-constructors'

type Foo = ADT<{
  bar: { value: string };
  baz: {};
}>

const id = <A>(a: A): A => a

const Foo = constructors<Foo>()(['bar', 'baz'])

const bar = Foo.bar({ value: 'zot' })

If you want smart constructors which can check the incoming data before constructing the value, it's a bit more work but it is supported:

import type { MakeADT } from 'ts-adt'
import { makeConstructors, some, none } from 'adt-constructors'

type Numbers = MakeADT<
  '_tag',
  {
    positive: { num: number };
    int: { int: number };
    float: { float: number };
  }
>

const Numbers =  makeConstructors('_tag')<Numbers>()(['float'], {
  positive: ({ num }) => num > 0 ? some({ num }) : none,
  int: ({ int }) => Math.floor(int) === int ? some({ int }) : none,
})

Notice that one of the options in the example above is still a simple constructor; they can be mixed! However, this is also fully type-checked, so any tags you don't include in the simple constructor list must be defined in the smart constructor object.

Note: The some function and none value are compatible with the Option type from fp-ts. You do not need to use fp-ts to use the type, though; it is simple enough to just redefine inside this library and avoid the fp-ts dependency. However, fp-ts includes a number of utilities that will probably be useful if you have smart constructors, such as fromPredicate. The Option type is discriminated on its _tag key, which will be either 'Some' or 'None'.

Generics

ADTs with generics aren't very well supported. Unfortunately, Typescript doesn't yet have a great way to deal with higher-kinded types, so you essentially need to specify the types in advance:

import type { ADT } from 'ts-adt'
import { constructors } from 'adt-constructors'

type Either<L, R> = ADT<{
  left: { left: L };
  right: { right: R };
}>

const Either = <L, R>() => constructors<Either<L, R>>()(['left', 'right'])

const EitherNumberString = Either<number, string>()
const num = EitherNumberString.left({ left: 3 })
const str = EitherNumberString.right({ right: 'foo' })

Depending on your use case, this might be fine, or you might need to write out your own constructors. For example, the above might be:

import type { ADT } from 'ts-adt'
import { constructors } from 'adt-constructors'

type Either<L, R> = ADT<{
  left: { left: L };
  right: { right: R };
}>

const left = <L, R=never>(left: L): Either<L, R> => ({ _type: 'left', left })
const right = <R, L=never>(right: R): Either<L, R> => ({ _type: 'right', right })

const num = left<number, string>(3)
const str = right<string, number>('foo')

Check out the tests for more examples.

[^1]: See here for more information.