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

// See the 'F# Tutorial' project for more help.

open System

​

let toInt a = 

    match (Int32.TryParse(a)) with

    | (true, x) -> Some x

    | (false, _) -> None

​

type MaybeBuilder() = 

    member this.Bind(x, f) = 

        printfn "x: %A, f: %A" x f

        match x with

        | None -> None

        | Some a -> f a

    member this.Return(x) = Some x

​

let maybe = new MaybeBuilder()

​

let tryHead arr = 

    if Array.isEmpty arr then None

    else Array.get arr 0 |> Some

​

let argv = [| "18" |]

​

let parsed = maybe { let! head = tryHead argv

                     let! v = head |> toInt

                     return v 

                    }

let parsedTwo = 

    maybe.Bind

        (tryHead argv, 

         fun head -> 

             maybe.Bind(toInt head, fun result -> 

                 maybe.Return(result)

             )

         )

​

printfn "parsed is %A" parsed

printfn "parsed2 is %A" parsedTwo

0 // return an integer exit code

​