Stream (FlowStream)

FlowStream<'env, 'error, 'value> represents a cold, pull-based asynchronous stream. Like Flow, a FlowStream requires an environment to run and can fail with a typed error. It is built on top of .NET’s IAsyncEnumerable, providing native support for backpressure and asynchronous iteration.

Note: FlowStream is currently available on .NET only.

Creating a Stream

From a Sequence

The simplest way to create a stream is from an existing seq.

let numbers = FlowStream.fromSeq [ 1 .. 10 ]

Transforming Streams

Mapping Values

You can transform the successful values in a stream using FlowStream.map.

let doubled = 
    numbers 
    |> FlowStream.map (fun x -> x * 2)

Consuming Streams

To consume a stream, you use one of the execution helpers. These helpers turn the stream into a Flow that you can then run or compose.

runForEach

Executes an action for every successful value in the stream. If the stream encounters a failure, execution stops and the Flow returns that failure.

let printNumbers =
    FlowStream.runForEach () (printfn "Value: %d") numbers

Why use FlowStream?

FlowStream is designed for scenarios where you need to process large amounts of data without loading everything into memory at once. Because it is part of the FsFlow family, it integrates perfectly with your existing environments, errors, and cancellation logic.

• Environment-Aware: Can read dependencies like databases or APIs during iteration.
• Typed Failures: Handles errors consistently with the rest of your application.
• Cancellable: Automatically respects the CancellationToken provided to Flow.run.

API Reference: Module FlowStream

Next Steps

Explore more advanced concurrency patterns in State and Concurrency.