type Future<'a> () =

    let mutable _value: 'a option = None

​

    member x.Resolve value =

        if _value.IsSome then failwith "can only resolve once"

        _value <- Some value

​

    member x.IsResolved with get () = _value.IsSome

​

    member x.Value 

        with get () = 

            match _value with 

            | Some v -> v

            | None -> failwith "hasn't resolved yet"

let testHttpGet response =

    let urlFuture = Future<string>()

​

    let get url = 

        urlFuture.Resolve url

        response

​

    (get, urlFuture)

let get, urlFuture = testHttpGet "resp"

​

let result = get "http://test"

​

printfn "result %s" result

​

printfn "future %s" urlFuture.Value

​