Program.fs

// Learn more about F# at http://fsharp.org

open System

type OverloadMeths =
    static member Map(m: 'T option, f) = Option.map f m
    static member Map(m: 'T when 'T:null, f) = m |> Option.ofObj |> Option.map f
    //works in 5.0 if you comment out this line
    static member Map(m: 'T Nullable, f) = m |> Option.ofNullable |> Option.map f

type OptionBuilder() =
      member __.Zero() = None
      member __.Return(x: 'T) = Some x
      member __.Delay(f: unit -> _) = f
      member __.Run(f) = f()

      member __.Bind(m: 'T option, f) = Option.bind f m
      member __.Bind(m: 'T when 'T:null, f) = m |> Option.ofObj |> Option.bind f
      //works in 5.0 if you comment out this line
      member __.Bind(m: 'T Nullable, f) = m |> Option.ofNullable |> Option.bind f


let option = OptionBuilder()

[<AllowNullLiteral>]
type Node (child:Node)=
    new() = new Node(null)
    member val child:Node = child with get,set

[<EntryPoint>]
let main argv =

    let parent = Node()
    let b1 = OverloadMeths.Map(parent.child, fun x -> x.child)
    let c1 = OverloadMeths.Map(b1, fun x -> x.child)

    printfn "c1=%A" c1

    let parent = Node()

    let s2  = option {
         let! a2 = parent.child
         return a2
    }
    printfn "s2=%A" s2

    let d3 =
        option {
            let! a3 = parent.child
            let! b3 = a3.child
            let! c3 = b3.child
            return c3
        } 
    printfn "d3=%A" d3
    0 // return an integer exit code