map, bind, and unit for F# Async

async.fs

type Microsoft.FSharp.Control.Async with
    static member unit (t:'T) : Async<'T> = async { return t }
    static member bind (fn:'T -> Async<'U>) (t: Async<'T>) : Async<'U> = 
        async {
            let! input = t
            return! fn(input)
        }
    static member map (fn:'T -> 'U) (t:Async<'T>) : Async<'U> =
        async {
            let! input = t
            return fn(input)
        }