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

# 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.

## Declaration

```swift
func tryScan<T>(_ initialResult: T, _ nextPartialResult: @escaping (T, Self.Output) throws -> T) -> Publishers.TryScan<Self, T>
```

## 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

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

Discussion Use 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(_:_:) calls a division function on elements of a collection publisher. The 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.

## See Also

### Mapping elements

- [map(_:)](combine/publisher/map(_:)-99evh.md)
- [tryMap(_:)](combine/publisher/trymap(_:).md)
- [mapError(_:)](combine/publisher/maperror(_:).md)
- [replaceNil(with:)](combine/publisher/replacenil(with:).md)
- [scan(_:_:)](combine/publisher/scan(_:_:).md)
- [setFailureType(to:)](combine/publisher/setfailuretype(to:).md)