import type { Ary } from ".";
import type { Args, GenericFn, GenericResolver } from "../HKT";

/**
 * Mix a type with the element type of an {@link Ary} (i.e., `Array` or `ReadonlyArray`).
 *
 * Sig: `<T, U>(y: U, a: Ary<T>) => Ary<T | U>`
 *
 * @example
 * ```typescript
 * type R1 = Mix<number, string[]>;
 * //   ^?: (string | number)[]
 * type R2 = Mix<"foo", readonly number[] >;
 * //   ^?: readonly (number | "foo")[]
 * ```
 */
export type Mix<U, A extends Ary> =
  A extends unknown[] ? Array<A[number] | U> : ReadonlyArray<A[number] | U>;

interface Resolver extends GenericResolver<[unknown, Ary<unknown>], Ary<unknown>> {
  on1_: ([y]: Args<this>) => [[Ary<unknown>], Ary<unknown>];
  on_1: ([, a]: Args<this>) => [[unknown], Ary<unknown>];
  on11: ([y, a]: Args<this>) => [[], Mix<typeof y, typeof a>];
}

/**
 * [Fn] Mix a type with the element type of an {@link Ary} (i.e., `Array` or `ReadonlyArray`).
 *
 * Sig: `<T, U>(y: U, a: Ary<T>) => Ary<T | U>`
 */
export default interface MixFn extends GenericFn<Resolver> {
  def: ([y, a]: Args<this>) => Mix<typeof y, typeof a>;
}