rivo/List/Init.d.ts

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import type { List } from ".";
import type { Args, GenericFn, GenericResolver } from "../HKT";
import type { None, Option, Some } from "../Option";

/**
 * Get all elements in a {@link List} except the last one.
 *
 * Sig: `<T>(xs: List<T>) => Option<List<T>>`
 */
export type Init<AS extends List> =
  AS extends readonly [...infer Init, unknown] ? Some<Init> : None;

interface Resolver extends GenericResolver<[List], Option<List<unknown>>> {
  on1: ([xs]: Args<this>) => [
    [],
    Option<typeof xs extends unknown[] ? (typeof xs)[number][] : List<(typeof xs)[number]>>,
  ];
}

/**
 * Get all elements in a {@link List} except the last one.
 *
 * Sig: `<T>(xs: List<T>) => Option<List<T>>`
 */
export default interface InitFn extends GenericFn<Resolver> {
  def: ([xs]: Args<this>) => Init<typeof xs>;
}