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# Publishers.FlatMap

A publisher that transforms elements from an upstream publisher into a new publisher.

```
struct FlatMap<NewPublisher, Upstream> where NewPublisher : Publisher, Upstream : Publisher, NewPublisher.Failure == Upstream.Failure
```

## Topics

### Creating a flat map Publisher

[`init(upstream:maxPublishers:transform:)`](/documentation/Combine/Publishers/FlatMap/init(upstream:maxPublishers:transform:))

Creates a publisher that transforms elements from an upstream publisher into a new publisher.

### Declaring supporting types

[`Publishers.FlatMap.Output`](/documentation/Combine/Publishers/FlatMap/Output)

The kind of values published by this publisher.

[`Publishers.FlatMap.Failure`](/documentation/Combine/Publishers/FlatMap/Failure)

The kind of errors this publisher might publish.

### Inspecting publisher properties

[`upstream`](/documentation/Combine/Publishers/FlatMap/upstream)

The publisher from which this publisher receives elements.

[`maxPublishers`](/documentation/Combine/Publishers/FlatMap/maxPublishers)

The maximum number of concurrent publisher subscriptions

[`transform`](/documentation/Combine/Publishers/FlatMap/transform)

A closure that takes an element as a parameter and returns a publisher that produces elements of that type.


---

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