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

/**
 * Get the element type of an {@link Ary} (i.e., `Array` or `ReadonlyArray`).
 *
 * Sig: `<T>(a: Ary<T>) => T`
 *
 * @example
 * ```typescript
 * type R1 = Elem<readonly string[]>;
 * //   ^?: string
 * type R2 = Elem<(number | boolean)[]>;
 * //   ^?: number | boolean
 * type R3 = Elem<[]>;
 * //   ^?: never
 * type R4 = Elem<[1, 2, 3]>;
 * //   ^?: 1 | 2 | 3
 * ```
 */
export type Elem<A extends Ary> = A[number];

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

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