# reduce(_:_:) Applies a closure that collects each element of a stream and publishes a final result upon completion. ``` func reduce(_ initialResult: T, _ nextPartialResult: @escaping (T, Self.Output) -> T) -> Publishers.Reduce ``` ## Parameters `initialResult` The value that the closure receives the first time it’s called. `nextPartialResult` A closure that produces a new value by taking the previously-accumulated value and the next element it receives from the upstream publisher. ## Return Value A publisher that applies the closure to all received elements and produces an accumulated value when the upstream publisher finishes. If [`reduce(_:_:)`](/documentation/Combine/Publisher/reduce(_:_:)) receives an error from the upstream publisher, the operator delivers it to the downstream subscriber, the publisher terminates and publishes no value. ## Discussion Use [`reduce(_:_:)`](/documentation/Combine/Publisher/reduce(_:_:)) to collect a stream of elements and produce an accumulated value based on a closure you provide. In the following example, the [`reduce(_:_:)`](/documentation/Combine/Publisher/reduce(_:_:)) operator collects all the integer values it receives from its upstream publisher: ``` let numbers = (0...10) cancellable = numbers.publisher .reduce(0, { accum, next in accum + next }) .sink { print("\($0)") } // Prints: "55" ``` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)