Async Programming

Async Workflows

let fetchUrlAsync url = 
    async {
        let! html = HttpClient.GetStringAsync(url) 
                    |> Async.AwaitTask
        return html.Length
    }

let lengths = 
    ["http://example.com"; "http://fsharp.org"]
    |> List.map fetchUrlAsync
    |> Async.Parallel
    |> Async.RunSynchronously

Tasks

let computeTask() = task {
    let! result1 = Task.Run(fun () -> 2 + 2)
    let! result2 = Task.Run(fun () -> 3 * 3)
    return result1 + result2
}

let finalResult = computeTask().Result

Agents (MailboxProcessor)

type Message = 
    | Increment of int
    | GetCount of AsyncReplyChannel

let counter = MailboxProcessor.Start(fun inbox ->
    let rec loop count = async {
        let! msg = inbox.Receive()
        match msg with
        | Increment n -> return! loop (count + n)
        | GetCount reply -> 
            reply.Reply(count)
            return! loop count
    }
    loop 0)