rivo/List/Prepend.d.ts

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import type { List } from ".";
import type { Args, GenericFn, GenericResolver } from "../HKT";
import type { Broaden } from "../helpers";

/**
 * Prepend an element to a {@link List} (i.e., fixed-length tuple).
 *
 * Sig: `<T>(x: T, xs: List<T>) => List<T>`
 */
export type Prepend<T, TS extends List> = TS extends unknown[] ? [T, ...TS] : readonly [T, ...TS];

interface Resolver extends GenericResolver<[unknown, List], List> {
  on1_: ([x]: Args<this>) => [[List<Broaden<typeof x>>], List<Broaden<typeof x>>];
  on_1: ([, xs]: Args<this>) => [
    [Broaden<(typeof xs)[number]>],
    List<Broaden<(typeof xs)[number]>>,
  ];
  on11: ([x, xs]: Args<this>) => [[], List<Broaden<typeof x>>];
}

/**
 * [Fn] Prepend an element to a {@link List} (i.e., fixed-length tuple).
 *
 * Sig: `<T>(x: T, xs: List<T>) => List<T>`
 */
export default interface PrependFn extends GenericFn<Resolver> {
  def: ([x, xs]: Args<this>) => Prepend<typeof x, typeof xs>;
}