import type { AnyIterable, AsyncReducer, Reducer } from "../../types";
/**
 * Reduces an input iterable to a sequence of accumulated values using an asynchronous reducer function.
 *
 * @group Lazy helpers
 * @template T - The type of elements in the input iterable.
 * @template U - The type of the output value. Defaults to `T`.
 * @param reducer - An asynchronous reducer function that takes the accumulator, a value, and its index.
 * @param initialValue - Optional. The initial value of the accumulator. If not provided, the first value of the iterable is used.
 * @returns A function that accepts an input iterable and returns an async iterable that yields accumulated values.
 *
 * @example
 * ```ts
 * const input = [1, 2, 3];
 * const add = async (acc: number, value: number) => acc + value;
 * const scanSum = scan(add)(input);
 *
 * (async () => {
 *   for await (const value of scanSum) {
 *     console.log(value); // Logs 1, 3, 6
 *   }
 * })();
 * ```
 */
export declare function scan<T, U = T>(reducer: AsyncReducer<T, U>, initialValue?: U): (input: AnyIterable<T>) => AsyncIterable<Awaited<U>>;
/**
 * Reduces an input sync iterable to a sequence of accumulated values using a reducer function.
 *
 * @group Lazy helpers
 * @template T - The type of elements in the input sync iterable.
 * @template U - The type of the output value. Defaults to `T`.
 * @param reducer - A reducer function that takes the accumulator, a value, and its index.
 * @param initialValue - Optional. The initial value of the accumulator. If not provided, the first value of the iterable is used.
 * @returns A function that accepts an input sync iterable and returns an iterable that yields accumulated values.
 *
 * @example
 * ```ts
 * const input = [1, 2, 3];
 * const add = (acc: number, value: number) => acc + value;
 * const scanSumSync = scanSync(add)(input);
 *
 * for (const value of scanSumSync) {
 *   console.log(value); // Logs 1, 3, 6
 * }
 * ```
 *
 * @remarks
 * Available as `scan` when imported from `peter-piper/sync`.
 */
export declare function scanSync<T, U = T>(reducer: Reducer<T, U>, initialValue?: U): (input: Iterable<T>) => Iterable<U>;