import type { List } from ".";
import type { Args, GenericFn, GenericResolver } from "../HKT";
import type { Nat } from "../Num";
import type { GTE } from "../Num/GTE";
import type { None, Option, Some } from "../Option";

/**
 * Get the element at a specific index in a {@link List}.
 *
 * Sig: `<T>(i: Nat, xs: List<T>) => Option<T>`
 */
export type At<I extends Nat, TS extends List> =
  GTE<I, TS["length"]> extends true ? None : Some<TS[I]>;

interface Resolver extends GenericResolver<[Nat, List], Option<unknown>> {
  on1_: ([i]: Args<this>) => [[List], Option<unknown>];
  on_1: ([, xs]: Args<this>) => [[Nat], Option<(typeof xs)[number]>];
  on11: ([i, xs]: Args<this>) => [[], Option<(typeof xs)[number]>];
}

/**
 * Get the element at a specific index in a {@link List}.
 *
 * Sig: `<T>(i: Nat, xs: List<T>) => Option<T>`
 */
export default interface AtFn extends GenericFn<Resolver> {
  def: ([i, xs]: Args<this>) => At<typeof i, typeof xs>;
}