import { Mapper } from '@naturalcycles/js-lib';
import { TransformOpt, TransformTyped } from '../stream.model';
/**
 * Similar to RxJS `tap` - allows to run a function for each stream item, without affecting the result.
 * Item is passed through to the output.
 *
 * Can also act as a counter, since `index` is passed to `fn`
 */
export declare function transformTap<IN>(fn: Mapper<IN, any>, opt?: TransformOpt): TransformTyped<IN, IN>;