import type { ReadOnlyArray, Unary } from "@lou.codes/types";
/**
 * Groups values of an iterable in an object based on the output of the
 * `grouper` function.
 *
 * @category Reducers
 * @example
 * ```typescript
 * const groupByType = groupBy((value: number) => number % 2 === 0 ? "even" : "odd");
 * groupByType([1, 2, 3, 4, 5]); // { even: [2, 4], odd: [1, 3, 5] }
 * ```
 * @param grouper Grouper function.
 * @returns Object with grouped values.
 */
export declare const groupBy: <Item, Key extends PropertyKey>(
	grouper: Unary<Item, Key>,
) => (
	iterable: Readonly<Iterable<Item>>,
) => Readonly<Record<Key, ReadOnlyArray<Item>>>;