What would be an F# equivalent of the C# code using async?

Question

I am struggling to write an F# code that would sequentially await for some asynchronous method calls. I am familiar with F# async workflows but can't figure out how to map to it a simple case.

Let's take as an example an async XmlReader. Here's how the C# code might look:

using (XmlReader r = XmlReader.Create(new StringReader(input), new XmlReaderSettings() { Async = true }))
{
    while (await r.ReadAsync())
    {
        switch (r.NodeType)
        {
            case XmlNodeType.Element:
                Console.WriteLine(r.LocalName);
                break;

            case XmlNodeType.Text:
                Console.WriteLine(await r.GetValueAsync());
                break;
        }
    }
}

If this code didn't use async calls, we could just rewrite it in F# using recursion and pattern matching. But it uses ReadAsync and GetValueAsync, how they can be expressed in F# counterpart?

why not just use recursion? I don't see anything that would prevent it — N_A
– N_A, Commented Feb 6, 2015 at 17:12
A C# to F# conversion (done badly) here: github.com/xamarin/xamarin-store-app/blob/master/Shared/Data/… github.com/xamarin/shirt-store-fsharp/blob/master/Shared/… — N_A
– N_A, Commented Feb 6, 2015 at 17:14

N_A · Accepted Answer · 2015-02-06 18:45:49Z

6

This is completely untested, but I think gets the point across. The essence of it is that you need to convert Task to Async using Async.AwaitTask then the rest of it is pretty obvious.

let doRecursiveAsyncThing input = async {
    use r = XmlReader.Create(new StringReader(input), new XmlReaderSettings(Async = true ))
    let loop x = async {
        let! noteType = r.ReadAsync() |> Async.AwaitTask
        match noteType with
        | XmlNodeType.Element -> Console.WriteLine r.LocalName
                                 do! loop x
        | XmlNodeType.Text    -> let! value = r.GetValueAsync() |> Async.AwaitTask
                                 Console.WriteLine value
                                 do! loop x
        | _                   -> () }

    }
    do! loop r

edited Feb 6, 2015 at 18:45

answered Feb 6, 2015 at 17:26

N_A

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4 Comments

Vagif Abilov Over a year ago

Thanks a lot! This is what I was trying to achieve. I missed Async.AwaitTask in my attempts to write similar code.

N_A Over a year ago

Note the helper function for async here: github.com/xamarin/shirt-store-fsharp/blob/master/Shared/…

Bent Tranberg Over a year ago

The link is dead.

N_A Over a year ago

@BentTranberg github.com/xamarin/FSXamarinStore/blob/master/Shared/…

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