import type { PipeStep } from './pipe$.js';
import type { AnyAsyncIterable, AnyAwaitableIterable, AnyIterable, AsyncIteratedValue, IteratedValue } from './types/inputs/MinimalIterator.js';
import type { AnySubscribable } from './types/inputs/Subscribable.js';
import type { Observable } from './types/outputs/Observable.js';
export type ReduceOrigin<D = unknown> = AnyAwaitableIterable<D> | AnySubscribable<D>;
export type ReduceOriginValue<O extends ReduceOrigin> = (O extends AnyIterable ? IteratedValue<O> : O extends AnyAsyncIterable ? AsyncIteratedValue<O> : unknown) & (O extends AnySubscribable<infer D> ? D : unknown);
export type ReduceCallback<T, S> = (state: S, item: T) => S;
export type ReduceResult<O, R> = O extends AnyIterable ? R : O extends AnyAsyncIterable ? Observable<R> : O extends AnySubscribable ? Observable<R> : never;
/**
 * Apply reducer-like function to each emitted value, emitting final result with origin completes.
 *
 * @since 1.0.0
 * @version 2.5.0 Add support for async iterators
 */
export declare function reduce$<O extends ReduceOrigin, const S>(cb: ReduceCallback<ReduceOriginValue<O>, S>, init: S): PipeStep<O, ReduceResult<O, S>>;