import type { List } from ".";
import type ConcatWFn from "./ConcatW";
import type { FoldL } from "./FoldL";
import type { Map } from "./Map";
import type { Args, Fn1, GenericFn, GenericResolver, Param0, Return } from "../HKT";

/**
 * Apply a function to each element of a {@link List} (i.e., fixed-length tuple) and concatenate the
 * results.
 *
 * Sig: `<T, U>(f: (x: T) => List<U>, xs: List<T>) => List<U>`
 */
export type FlatMap<F extends Fn1, TS extends List> = FoldL<
  ConcatWFn,
  TS extends unknown[] ? [] : readonly [],
  Map<F, TS>
>;

interface Resolver extends GenericResolver<[Fn1<never, List>, List], List> {
  on1_: ([f]: Args<this>) => [[List<Param0<typeof f>>], List<Return<typeof f>[number]>];
  on_1: ([, xs]: Args<this>) => [[Fn1<(typeof xs)[number]>], List];
  on11: ([f, xs]: Args<this>) => [[], List<Return<typeof f>[number]>];
}

/**
 * [Fn] Apply a function to each element of a {@link List} (i.e., fixed-length tuple) and
 * concatenate the results.
 *
 * Sig: `<T, U>(f: (x: T) => List<U>, xs: List<T>) => List<U>`
 */
export default interface FlatMapFn extends GenericFn<Resolver> {
  def: ([f, xs]: Args<this>) => FlatMap<typeof f, typeof xs>;
}