This is a sample implementation of Callable in F#.

Callable.fs

// Interface
type ICallable<'a, 'b> =
    abstract member callback: 'a -> 'b

// concrete class
type Callable<'a, 'b>(f: 'a -> 'b) =
    interface ICallable<'a, 'b> with
        member self.callback x = f x

// computation builder
type CallerBuilder() =
    member self.Bind(x: ICallable<'a, 'b>, rest) = x.callback |> rest
    member self.Zero() = ()

let caller = CallerBuilder()

// usage
caller {
    let! f = Callable(fun x -> x + 2)
    f 3 |> printfn "%d"
}
//=> 5