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

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

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

/**
 * [Fn] Get the last element in a {@link List}.
 *
 * Sig: `<T>(xs: List<T>) => Option<T>`
 */
export default interface LastFn extends GenericFn<Resolver> {
  def: ([xs]: Args<this>) => Last<typeof xs>;
}