# tryReduce(_:_:) Applies an error-throwing closure that collects each element of a stream and publishes a final result upon completion. ``` func tryReduce(_ initialResult: T, _ nextPartialResult: @escaping (T, Self.Output) throws -> T) -> Publishers.TryReduce ``` ## Parameters `initialResult` The value that the closure receives the first time it’s called. `nextPartialResult` An error-throwing closure that takes the previously-accumulated value and the next element from the upstream publisher to produce a new value. ## Return Value A publisher that applies the closure to all received elements and produces an accumulated value when the upstream publisher finishes. ## Discussion Use [`tryReduce(_:_:)`](/documentation/Combine/Publisher/tryReduce(_:_:)) to collect a stream of elements and produce an accumulated value based on an error-throwing closure you provide. If the closure throws an error, the publisher fails and passes the error to its subscriber. In the example below, the publisher’s `0` element causes the `myDivide(_:_:)` function to throw an error and publish the result: ``` struct DivisionByZeroError: Error {} func myDivide(_ dividend: Double, _ divisor: Double) throws -> Double { guard divisor != 0 else { throw DivisionByZeroError() } return dividend / divisor } var numbers: [Double] = [5, 4, 3, 2, 1, 0] numbers.publisher .tryReduce(numbers.first!, { accum, next in try myDivide(accum, next) }) .catch({ _ in Just(Double.nan) }) .sink { print("\($0)") } ``` --- 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)