# tryScan(_:_:) Transforms elements from the upstream publisher by providing the current element to an error-throwing closure along with the last value returned by the closure. ``` func tryScan(_ initialResult: T, _ nextPartialResult: @escaping (T, Self.Output) throws -> T) -> Publishers.TryScan ``` ## Parameters `initialResult` The previous result returned by the `nextPartialResult` closure. `nextPartialResult` An error-throwing closure that takes as its arguments the previous value returned by the closure and the next element emitted from the upstream publisher. ## Return Value A publisher that transforms elements by applying a closure that receives its previous return value and the next element from the upstream publisher. ## Discussion Use [`tryScan(_:_:)`](/documentation/Combine/Publisher/tryScan(_:_:)) to accumulate all previously-published values into a single value, which you then combine with each newly-published value. If your accumulator closure throws an error, the publisher terminates with the error. In the example below, [`tryScan(_:_:)`](/documentation/Combine/Publisher/tryScan(_:_:)) calls a division function on elements of a collection publisher. The [`Publishers.TryScan`](/documentation/Combine/Publishers/TryScan) publisher publishes each result until the function encounters a `DivisionByZeroError`, which terminates the publisher. ``` struct DivisionByZeroError: Error {} /// A function that throws a DivisionByZeroError if `current` provided by the TryScan publisher is zero. func myThrowingFunction(_ lastValue: Int, _ currentValue: Int) throws -> Int { guard currentValue != 0 else { throw DivisionByZeroError() } return (lastValue + currentValue) / currentValue } let numbers = [1,2,3,4,5,0,6,7,8,9] cancellable = numbers.publisher .tryScan(10) { try myThrowingFunction($0, $1) } .sink( receiveCompletion: { print ("\($0)") }, receiveValue: { print ("\($0)", terminator: " ") } ) // Prints: "11 6 3 1 1 -1 failure(DivisionByZeroError())". ``` If the closure throws an error, the publisher fails with the error. --- 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)