import { Iterator } from "../../lazy";
import type { AnyObject, Options } from "../../types";
interface InputExpr {
    /** The output collection. */
    readonly into: string | AnyObject[];
    /** Field or fields that act as a unique identifier for a document. */
    readonly on?: string | string[];
    /** Specifies variables for use in the whenMatched pipeline. */
    readonly let?: AnyObject;
    /**
     * The behavior of $merge if a result document and an existing document
     * in the collection have the same value for the specified on field(s).
     */
    readonly whenMatched?: "replace" | "keepExisting" | "merge" | "fail" | AnyObject[];
    /** The behavior of $merge if a result document does not match an existing document in the out collection. */
    readonly whenNotMatched?: "insert" | "discard" | "fail";
}
/**
 * Writes the resulting documents of the aggregation pipeline to a collection.
 *
 * NB: Object are deep cloned for outputing regardless of the ProcessingMode.
 *
 * See {@link https://www.mongodb.com/docs/manual/reference/operator/aggregation/merge/ usage}.
 */
export declare function $merge(coll: Iterator, expr: InputExpr, options: Options): Iterator;
export {};