---
title: "tryCompactMap(_:)"
framework: combine
role: symbol
role_heading: Instance Method
path: "combine/publisher/trycompactmap(_:)"
---

# tryCompactMap(_:)

Calls an error-throwing closure with each received element and publishes any returned optional that has a value.

## Declaration

```swift
func tryCompactMap<T>(_ transform: @escaping (Self.Output) throws -> T?) -> Publishers.TryCompactMap<Self, T>
```

## Parameters

- `transform`: An error-throwing closure that receives a value and returns an optional value.

## Return Value

Return Value Any non-nil optional results of calling the supplied closure.

## Discussion

Discussion Use 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. The following example uses an array of numbers as the source for a collection-based publisher. A 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 String. If the closure called by tryCompactMap(_:) fails to look up a Roman numeral, it returns the optional String (unknown). If the closure called by tryCompactMap(_:) determines the input is 0, it throws an error. The tryCompactMap(_:) operator catches this error and stops publishing, sending a 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())"

## See Also

### Filtering elements

- [filter(_:)](combine/publisher/filter(_:).md)
- [tryFilter(_:)](combine/publisher/tryfilter(_:).md)
- [compactMap(_:)](combine/publisher/compactmap(_:).md)
- [removeDuplicates()](combine/publisher/removeduplicates().md)
- [removeDuplicates(by:)](combine/publisher/removeduplicates(by:).md)
- [tryRemoveDuplicates(by:)](combine/publisher/tryremoveduplicates(by:).md)
- [replaceEmpty(with:)](combine/publisher/replaceempty(with:).md)
- [replaceError(with:)](combine/publisher/replaceerror(with:).md)