// @ts-ignore - Only used in doc comments
import type { Reject } from "./Reject";
// @ts-ignore - Only used in doc comments
import type RejectFn from "./Reject";
import type { Args, Call1W, Fn1, GenericFn, GenericResolver, Param0 } from "../HKT";

/**
 * Filter a union based on a predicate.
 *
 * Sig: `<T>(pred: (x: T) => boolean, u: UnionOf<T>) => UnionOf<T>`
 *
 * @see {@link Reject} for the opposite.
 */
export type Filter<Pred extends Fn1<never, boolean>, U> =
  U extends U ?
    Call1W<Pred, U> extends true ?
      U
    : never
  : never;

interface Resolver extends GenericResolver<[Fn1<never, boolean>, unknown], unknown> {
  on1_: ([pred]: Args<this>) => [[Param0<typeof pred>], Param0<typeof pred>];
  on_1: ([, u]: Args<this>) => [[Fn1<typeof u, boolean>], typeof u];
  on11: ([pred, u]: Args<this>) => [[], typeof u];
}

/**
 * [Fn] Filter a union based on a predicate.
 *
 * Sig: `<T>(pred: (x: T) => boolean, u: UnionOf<T>) => UnionOf<T>`
 *
 * @see {@link RejectFn} for the opposite.
 */
export default interface FilterFn extends GenericFn<Resolver> {
  def: ([pred, u]: Args<this>) => Filter<typeof pred, typeof u>;
}