open System

type Maybe<'T> = Just of 'T | Nothing

let bind ma f =
    match ma with
    | Just x -> f x
    | Nothing -> Nothing

type MaybeBuilder() =
    member x.Bind(v,f) = bind v f
    member x.Return v = Just v
    member x.ReturnFrom o = o
    member x.Zero() = Nothing

let maybe = MaybeBuilder()

let targetFunc (str:string) (date:DateTime) (amount: decimal) (ok:bool) : Maybe<decimal> =
    Console.WriteLine("{0} {1} {2} {3}", str, date, amount, ok)
    if ok
        then Just(amount + 50m)
        else Nothing

let testFunc (mAmount:Maybe<decimal>) : Maybe<decimal> =
    let mStr = Just("hi")
    let mDate = Just(DateTime.Now)
    let mOk = Just(true)

    maybe {
        let! str = mStr
        let! date = mDate
        let! amount = mAmount
        let! ok = mOk

        return! targetFunc str date amount ok
    }

let result = testFunc (Just 10m)