open System
open System.Net

type HttpListener with
    static member Run (url:string,handler: (HttpListenerRequest -> HttpListenerResponse -> Async<unit>)) = 
        let listener = new HttpListener()
        listener.Prefixes.Add url
        listener.Start()
        let asynctask = Async.FromBeginEnd(listener.BeginGetContext,listener.EndGetContext)
        async {
            while true do 
                let! context = asynctask
                Async.Start (handler context.Request context.Response)
        } |> Async.Start 
        listener

HttpListener.Run("http://*:80/App/",(fun req resp -> 
        async {
            let out = Text.Encoding.ASCII.GetBytes "hello world"
            resp.OutputStream.Write(out,0,out.Length)
            resp.OutputStream.Close()
        }
    )) |> ignore

Console.Read () |> ignore