typelevel-ts

Requires TypeScript v2.4.1+

Adapted from

• https://github.com/Microsoft/TypeScript/issues/14833
• https://github.com/Microsoft/TypeScript/issues/16392
• https://github.com/Microsoft/TypeScript/issues/12215

Naturals

Example. Type-safe vectors

import { _1, _2, _3, Nat, Add } from 'typelevel-ts'

function create<A>(as: [A, A, A]): Vector<_3, A>
function create<A>(as: [A, A]): Vector<_2, A>
function create<A>(as: [A]): Vector<_1, A>
function create<N extends Nat, A>(as: Array<A>): Vector<N, A> {
  return new Vector<N, A>(as)
}

class Vector<N extends Nat, A> {
  static create = create
  readonly _N: N
  readonly _A: A
  constructor(public value: Array<A>) {}
  append<N2 extends Nat>(vector: Vector<N2, A>): Vector<Add<N, N2>, A> {
    return new Vector<Add<N, N2>, A>(this.value.concat(vector.value))
  }
  zip<B>(vector: Vector<N, B>): Vector<N, [A, B]> {
    return new Vector<N, [A, B]>(this.value.map((a, i) => [a, vector.value[i]] as [A, B]))
  }
  inspect() {
    return this.toString()
  }
  toString() {
    return `Vector(${JSON.stringify(this.value)})`
  }
}

// v1 :: Vector<_1, number>
const v1 = Vector.create([1])
// v2 :: Vector<_2, number>
const v2 = Vector.create([2, 3])
// v3 :: Vector<_3, number>
const v3 = v1.append(v2)

// v3.zip(v2) // error

console.log(v2.zip(v1.append(v1))) // Vector([[2,1],[3,1]])

The ObjectOmit operator

Example. A withDefaults function (React)

import * as React from 'react'
import { ObjectOmit } from 'typelevel-ts'

export default function withDefaults<A, D extends keyof A>(
  C: React.ComponentType<A>,
  defaults: Pick<A, D>
): React.SFC<ObjectOmit<A, D> & Partial<Pick<A, D>>> {
  return (props: any) => <C {...Object.assign({}, defaults, props)} />
}

class Foo extends React.Component<{ bar: string; baz: number }, void> {}
const DefaultedFoo = withDefaults(Foo, { baz: 1 })
const x = <DefaultedFoo bar="bar" /> // ok

Example. A withProps function (React)

import { ObjectOmit } from 'typelevel-ts'
import * as React from 'react'

function withProps<D, P extends D>(C: React.ComponentType<P>, values: D): React.SFC<ObjectOmit<P, keyof D>> {
  return (props: any) => <C {...Object.assign({}, props, values)} />
}

class Foo extends React.Component<{ bar: string; baz: number }, void> {}
const FilledFoo = withProps(Foo, { baz: 1 })
const x = <FilledFoo bar="bar" /> // ok

The ObjectDiff operator

import { Diff } from 'typelevel-ts'

type Foo = { a: string; b: number }

type Bar = { b: number }

// Baz :: { a: string, b?: number }
type Baz = Diff<Foo, Bar>