type Result<'a> = 

    | Success of 'a

    | Failure of string

​

type ResultBuilder() =

    member m.Return a = Success(a)

    member m.Bind(r,fn) =

        match r with

        | Success(a) -> fn a

        | Failure(m) -> Failure(m)

module Parser = 

    let res = ResultBuilder()

    let Combine p1 p2 fn = 

        fun a -> res { let! x = p1 a

                       let! y = p2 a

                       return fn(x,y) }

    let Combine2 p1 p2 fn =

        fun a -> res { let! x = p1 a

                       let! y = p2 a

                       try

                          return fn(x,y) 

                       with

                         | ex -> Failure(ex.Message) }