import type { List } from ".";
import type { FoldL } from "./FoldL";
import type { Args, Fn } from "../HKT";
import type { Int } from "../Num";
import type AddFn from "../Num/Int/Add";
import type { AssertInt } from "../helpers";

/**
 * Calculate the sum of a {@link List} of {@link Int}s.
 *
 * Sig: `(ns: List<Int>) => Int`
 */
export type Sum<NS extends List<Int>> = AssertInt<
  FoldL<AddFn, 0, NS> extends infer R extends Int ? R : never
>;

/**
 * [Fn] Calculate the sum of a {@link List} of {@link Int}s.
 *
 * Sig: `(ns: List<Int>) => Int`
 */
export default interface SumFn extends Fn<[List<Int>], Int> {
  def: ([ns]: Args<this>) => Sum<typeof ns>;
}