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

/**
 * Create a pair with one element.
 *
 * Sig: `<T>(x: T) => readonly [T, T]`
 *
 * @example
 * ```typescript
 * type R1 = Duplicate<1>;
 * //   ^?: readonly [1, 1]
 * type R2 = Duplicate<"foo" | false>;
 * //   ^?: readonly ["foo" | false, "foo" | false]
 * ```
 */
export type Duplicate<T> = readonly [T, T];

interface Resolver extends GenericResolver<[unknown], readonly [unknown, unknown]> {
  on1: ([x]: Args<this>) => [[], readonly [typeof x, typeof x]];
}

/**
 * [Fn] Create a tuple with one element.
 *
 * Sig: `<T>(x: T) => readonly [T]`
 */
export default interface DuplicateFn extends GenericFn<Resolver> {
  def: ([x]: Args<this>) => Duplicate<typeof x>;
}