タプルのrest要素、curryの型

概要

可変長タプルの基本

type HeadTail<A extends any[], L> = [...A, L]

function callWithLast<A extends any[], L, R>(fn: (...args: [...A, L]) => R, last: L, ...head: A): R {
  return fn(...head, last)
}

// Example
function join3(a: string, b: number, c: boolean) { return `${a}-${b}-${c}` }
const r = callWithLast(join3, true, "x", 42)   // ok → string

Curry ヘルパー

function curryLast<A extends any[], L, R>(fn: (...args: [...A, L]) => R) {
  return (last: L) => (...head: A) => fn(...head, last)
}

const curried = curryLast(join3)
const out = curried(true)("x", 42) // string

汎用的な curryFromFn

type Fn = (...args: any[]) => any

type Tail<T extends any[]> = T extends [...infer H, any] ? H : never

type Last<T extends any[]> = T extends [...any[], infer L] ? L : never

function curryFromFn<F extends Fn>(fn: F) {
  type A = Tail<Parameters<F>>
  type L = Last<Parameters<F>>
  return (last: L) => (...head: A) => fn(...head, last)
}

const cur = curryFromFn(join3)
const o2 = cur(true)("x", 42) // still string

タプルで組み合わせる

function compose<A extends any[], B, C>(g: (b: B) => C, f: (...args: A) => B) {
  return (...args: A): C => g(f(...args))
}

function len(a: string, b: number) { return `${a}${b}`.length }
function isEven(n: number) { return n % 2 === 0 }

const comp = compose(isEven, len)
const ok = comp("id", 99) // boolean

ヒント

Curry シグネチャの確認

振り返り