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type Tuple<Iterables extends Iterable<any>[]> = {
  [K in keyof Iterables]: Iterables[K] extends Iterable<infer A> ? A : never
}

export const product = <Iterables extends Iterable<any>[]>(...t: Iterables): Iterable<Tuple<Iterables>> =>
  t.length === 0 ? [] : prod(t, [])

export function* prod<Iterables extends Iterable<any>[]>([h, ...t]: Iterables, r: any[]): Iterable<Tuple<Iterables>> {
  if (t.length === 0) for (const a of h) yield [...r, a] as Tuple<Iterables>
  else for (const a of h) yield* prod(t, [...r, a]) as Iterable<Tuple<Iterables>>

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import { co, unsafeRun, resumeLater, resumeNow, use, Resume, op } from '../src'
import { EOL } from 'os'
import { createInterface } from 'readline'

type Print = { print(s: string): Resume<void> }
const print = (s: string) => op<Print>(c => c.print(s))

type Read = { read(): Resume<string> }
const read = op<Read>(c => c.read())

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Simpler Variadic Function Types in TypeScript (part 2): Higher-order Variadic Functions

In part 1 we looked at a technique that allows expressing strongly-typed variadic functions in a way that solves problems with the current most common approach. It reduces code size and repetition, and it scales to any arity.

I mentioned that the technique can also be extended to higher-order variadic functions, such as the typical zipWith. Let’s explore how to do that.

ZipWith Example

As we did in part 1, let’s start with an example, a typical zipWith on arrays.

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Simpler Variadic Function Types in TypeScript (part 1)

Variadic functions, such as the common zip function on arrays, are convenient and remove the need for lots of specific arity-function variants, e.g., zip2, zip3, zip4, etc. However, they can be difficult and tedious to type correctly in TypeScript when the return type depends on the parameter types, and the parameter types are heterogeneous.

Zip Example

Given a typical zip on arrays:

const a: number[] = [1, 2, 3]

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export type Cancel = void | ((k: (r: void) => void) => void)

export const runCancel = (c: Cancel, k: (r: void) => void): void =>
  c ? c(k) : k()

export type Fx<H, A> = (handler: H, k: (a: A) => void) => Cancel
export type Pure<A> = Fx<{}, A>

export const handle = <H0, H1, A>(fx: Fx<H0 & H1, A>, h1: H1): Fx<H0, A> =>
  (h0, k) => fx({ ...h0, ...h1 } as H0 & H1, k)

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// @flow

// Type-level lists
type Empty = []

// Type-level tail function
type Tail<L> = $Call<FTail, L>
interface FTail {
  (Empty): Empty,
  <H, T>([H, T]): T

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// @flow

// Type-level naturals
type Zero = 0

interface Succ {
  <N>(N): [1, N]
}

// Hey look, we can do type-level pattern matching

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// 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)]

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// @flow
import { curry2, curry3 } from '@most/prelude'

export type Except<E, A> = Exception<E, A> | Result<E, A>

export const result = <E, A> (a: A): Except<E, A> =>
  new Result(a)

export const throwError = <E, A> (e: E): Except<E, A> =>
  new Exception(e)

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