Asyncing the F# Way

I recently gave a talk at the Winnipeg .Net User Group. This talk was focused on features of F# that are going to make their way into the world of C#. During the presentation I was talking about using local functions to wrap async functions. It was at this point someone asked if F# can do async/await. Well, the answer is yes.

The Async Builder

F# uses an Async Computation Expression for creating asynchronous workflows. The notation we use to denote an async computation is the async { ... }. Anything inside the curly braces are executed within the computation expression.

async {
    printfn "Good night"
    do! Async.Sleep 2000
    printfn "Good morning"
} |> Async.RunSynchronously

Looking at the example above, we have our async { ... } and our code within the curly braces. If you run the code in your REPL (Read Evaluate Print Loop) it will print “Good night”, wait 2 seconds, then print “Good morning”.

The Bang notation

The do! (do-bang) is our way of expressing we are evaluating an asynchronous expression. The do itself indicates we don’t care about the result. It’s the ! (bang) that the AsyncBuilder uses to unwrap the asynchronous expression’s result. It’s syntactic sugar to ensure our code is expressive and readable.

We use let! (let-bang) when we want to use the result of an asynchronous expression. In the doWorkAsync example below, we use the let! to bind the result of addAsync to the label result so we can print it.

let addAsync x y = async { return x + y }

let doWorkAsync = async {
    let! result = addAsync 4 6
    printfn "Result: %d" result

Async.RunSynchronously doWorkAsync

Binding IDisposable

When we have an asynchronous expression which returns an object implementing IDisposable we should bind the result with use!. This unwraps the asynchronous expression, binds the disposable object, and disposes of the object when it falls out of scope. An example of use! is when retrieving the contents asynchronously from a website.

open System.IO
open System.Net

let getUrlContentSizeAsync (url : string) = 
    async {
        let request   = WebRequest.Create (url)
        use! response = request.AsyncGetResponse ()
        use stream    = response.GetResponseStream ()
        use reader    = new StreamReader(stream)
        let contents  = reader.ReadToEnd ()
        return (url,contents.Length)

getUrl "" |> Async.RunSynchronously

In the code above we are using the use! to unwrap the IDisposable response object. This in turn performs an automatic clean up when the object is no longer required.


We write asynchronous code in F# using the AsyncBuilder. The builder itself is syntactic sugar allowing us to write clean and expressive code. We use the Bang notation to unwrap the asynchronous calls within the builder. There are do!, let!, and use! to handle the results of an asynchronous expression. We’ve currently only looked at running the asynchronous code one way using the Async.RunSynchronously. We’ll look at more ways of running our code in a future post.

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  1. Pingback: Running with Async | Shane Charles

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