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

# tryReduce(_:_:)

Applies an error-throwing closure that collects each element of a stream and publishes a final result upon completion.

## Declaration

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

## 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

Return Value A publisher that applies the closure to all received elements and produces an accumulated value when the upstream publisher finishes.

## Discussion

Discussion Use 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 nan 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)") }

## See Also

### Reducing elements

- [collect()](combine/publisher/collect().md)
- [collect(_:)](combine/publisher/collect(_:).md)
- [collect(_:options:)](combine/publisher/collect(_:options:).md)
- [Publishers.TimeGroupingStrategy](combine/publishers/timegroupingstrategy.md)
- [ignoreOutput()](combine/publisher/ignoreoutput().md)
- [reduce(_:_:)](combine/publisher/reduce(_:_:).md)