Fun with a Lazy type and pairs

lazy-pair.js

// Pair helpers

// Map the first element of a pair
const first = <A, B, C> (f: A => B, [a, c]: [A, C]): [B, C] =>
  [f(a), c]

// Map the second element of a pair
const second = <A, B, C> (f: B => C, [a, b]: [A, B]): [A, C] =>
  [a, f(b)]

// Lazy type

type Lazy<A> = () => A

const lazy = <A> (a: A): Lazy<A> =>
  () => a

const force = <A> (la: Lazy<A>): A =>
  la()

const mapLazy = <A, B> (f: A => B, la: Lazy<A>): Lazy<B> =>
  () => f(force(la))

const firstLazy = <A, B, C> (f: A => B, lp: Lazy<[A, C]>): Lazy<[B, C]> =>
  () => first(f, force(lp))

const secondLazy = <A, B, C> (f: B => C, lp: Lazy<[A, B]>): Lazy<[A, C]> =>
  () => second(f, force(lp))