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

/**
 * Flatten an {@link Option} of an {@link Option} into an {@link Option}.
 *
 * Sig: `<T>(o: Option<Option<T>>) => Option<T>`
 */
export type Flatten<O extends Option<Option<unknown>>> =
  O extends Some<infer O extends Option<unknown>> ? O : None;

interface Resolver extends GenericResolver<[Option<Option<unknown>>], Option<unknown>> {
  on1: ([o]: Args<this>) => [[], typeof o extends Option<Option<infer T>> ? Option<T> : never];
  on_r: ([, r]: Args<this>) => [Option<Option<typeof r extends Option<infer T> ? T : never>>];
}

/**
 * [Fn] Flatten an {@link Option} of an {@link Option} into an {@link Option}.
 *
 * Sig: `<T>(o: Option<Option<T>>) => Option<T>`
 */
export default interface FlattenFn extends GenericFn<Resolver> {
  def: ([o]: Args<this>) => Flatten<typeof o>;
}