# tryCompactMap(_:) Calls an error-throwing closure with each received element and publishes any returned optional that has a value. ``` func tryCompactMap(_ transform: @escaping (Self.Output) throws -> T?) -> Publishers.TryCompactMap ``` ## Parameters `transform` An error-throwing closure that receives a value and returns an optional value. ## Return Value Any non-`nil` optional results of calling the supplied closure. ## Discussion Use [`tryCompactMap(_:)`](/documentation/Combine/Publisher/tryCompactMap(_:)) to remove `nil` elements from a publisher’s stream based on an error-throwing closure you provide. If the closure throws an error, the publisher cancels the upstream publisher and sends the thrown error to the downstream subscriber as a [`Failure`](/documentation/Combine/Publisher/Failure). The following example uses an array of numbers as the source for a collection-based publisher. A [`tryCompactMap(_:)`](/documentation/Combine/Publisher/tryCompactMap(_:)) operator consumes each integer from the publisher and uses a dictionary to transform the numbers from its Arabic to Roman numerals, as an optional . If the closure called by [`tryCompactMap(_:)`](/documentation/Combine/Publisher/tryCompactMap(_:)) fails to look up a Roman numeral, it returns the optional String `(unknown)`. If the closure called by [`tryCompactMap(_:)`](/documentation/Combine/Publisher/tryCompactMap(_:)) determines the input is `0`, it throws an error. The [`tryCompactMap(_:)`](/documentation/Combine/Publisher/tryCompactMap(_:)) operator catches this error and stops publishing, sending a [`Subscribers.Completion.failure(_:)`](/documentation/Combine/Subscribers/Completion/failure(_:)) that wraps the error. ``` struct ParseError: Error {} func romanNumeral(from: Int) throws -> String? { let romanNumeralDict: [Int : String] = [1: "I", 2: "II", 3: "III", 4: "IV", 5: "V"] guard from != 0 else { throw ParseError() } return romanNumeralDict[from] } let numbers = [6, 5, 4, 3, 2, 1, 0] cancellable = numbers.publisher .tryCompactMap { try romanNumeral(from: $0) } .sink( receiveCompletion: { print ("\($0)") }, receiveValue: { print ("\($0)", terminator: " ") } ) // Prints: "(Unknown) V IV III II I failure(ParseError())" ``` --- 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)