Asynchronous Workflows (F#)

This topic describes support in F# for performing computations asynchronously, that is, without blocking execution of other work. For example, asynchronous computations can be used to write applications that have UIs that remain responsive to users as the application performs other work.

async { expression }


In the previous syntax, the computation represented by expression is set up to run asynchronously, that is, without blocking the current computation thread when asynchronous sleep operations, I/O, and other asynchronous operations are performed. Asynchronous computations are often started on a background thread while execution continues on the current thread. The type of the expression is Async<'a>, where 'a is the type returned by the expression when the return keyword is used. The code in such an expression is referred to as an asynchronous block, or async block.

There are a variety of ways of programming asynchronously, and the Async class provides methods that support several scenarios. The general approach is to create Async objects that represent the computation or computations that you want to run asynchronously, and then start these computations by using one of the triggering functions. The various triggering functions provide different ways of running asynchronous computations, and which one you use depends on whether you want to use the current thread, a background thread, or a .NET Framework task object, and whether there are continuation functions that should run when the computation finishes. For example, to start an asynchronous computation on the current thread, you can use Async.StartImmediate. When you start an asynchronous computation from the UI thread, you do not block the main event loop that processes user actions such as keystrokes and mouse activity, so your application remains responsive.

Asynchronous Binding by Using let!

In an asynchronous workflow, some expressions and operations are synchronous, and some are longer computations that are designed to return a result asynchronously. When you call a method asynchronously, instead of an ordinary let binding, you use let!. The effect of let! is to enable execution to continue on other computations or threads as the computation is being performed. After the right side of the let! binding returns, the rest of the asynchronous workflow resumes execution.

The following code shows the difference between let and let!. The line of code that uses let just creates an asynchronous computation as an object that you can run later by using, for example, Async.StartImmediate or Async.RunSynchronously. The line of code that uses let! starts the computation, and then the thread is suspended until the result is available, at which point execution continues.

// let just stores the result as an asynchronous operation.
let (result1 : Async<byte[]>) = stream.AsyncRead(bufferSize)
// let! completes the asynchronous operation and returns the data.
let! (result2 : byte[])  = stream.AsyncRead(bufferSize)

In addition to let!, you can use use! to perform asynchronous bindings. The difference between let! and use! is the same as the difference between let and use. For use!, the object is disposed of at the close of the current scope. Note that in the current release of the F# language, use! does not allow a value to be initialized to null, even though use does.

Asynchronous Primitives

A method that performs a single asynchronous task and returns the result is called an asynchronous primitive, and these are designed specifically for use with let!. Several asynchronous primitives are defined in the F# core library. Two such methods for Web applications are defined in the module Microsoft.FSharp.Control.WebExtensions: WebRequest.AsyncGetResponse and WebClient.AsyncDownloadString. Both primitives download data from a Web page, given a URL. AsyncGetResponse produces a WebResponse object, and AsyncDownloadString produces a string that represents the HTML for a Web page.

Several primitives for asynchronous I/O operations are included in the Microsoft.FSharp.Control.CommonExtensions module. These extension methods of the Stream class are Stream.AsyncRead and Stream.AsyncWrite.

Additional asynchronous primitives are available in the F# PowerPack. You can also write your own asynchronous primitives by defining a function whose complete body is enclosed in an async block.

To use asynchronous methods in the .NET Framework that are designed for other asynchronous models with the F# asynchronous programming model, you create a function that returns an F# Async object. The F# library has functions that make this easy to do.

One example of using asynchronous workflows is included here; there are many others in the documentation for the methods of the Async class.


This example shows how to use asynchronous workflows to perform computations in parallel.

In the following code example, a function fetchAsync gets the HTML text returned from a Web request. The fetchAsync function contains an asynchronous block of code. When a binding is made to the result of an asynchronous primitive, in this case AsyncDownloadString, let! is used instead of let.

You use the function Async.RunSynchronously to execute an asynchronous operation and wait for its result. As an example, you can execute multiple asynchronous operations in parallel by using the Async.Parallel function together with the Async.RunSynchronously function. The Async.Parallel function takes a list of the Async objects, sets up the code for each Async task object to run in parallel, and returns an Async object that represents the parallel computation. Just as for a single operation, you call Async.RunSynchronously to start the execution.

The runAll function launches three asynchronous workflows in parallel and waits until they have all completed.

open System.Net
open Microsoft.FSharp.Control.WebExtensions

let urlList = [ "", "" 
                "MSDN", "" 
                "Bing", ""

let fetchAsync(name, url:string) =
    async { 
            let uri = new System.Uri(url)
            let webClient = new WebClient()
            let! html = webClient.AsyncDownloadString(uri)
            printfn "Read %d characters for %s" html.Length name
            | ex -> printfn "%s" (ex.Message);

let runAll() =
    |> fetchAsync
    |> Async.Parallel 
    |> Async.RunSynchronously
    |> ignore


See Also


Control.Async Class (F#)

Other Resources

F# Language Reference

Computation Expressions (F#)