namespace Case1

module MaybeBuilder =
    
    type MaybeBuilder() =
        member inline _.Bind(optionValue: 'T option, f) =
            match optionValue with
            | None -> None
            | Some value -> f value

        member inline _.Return maybeNull =
           if isNull maybeNull then None else Some maybeNull

        member inline _.Delay f = f

        member inline _.Run f = f()

    let maybe = MaybeBuilder()


module MaybeBuilderExtension =
    
    open MaybeBuilder
    
    type MaybeBuilder with
        member inline _.Bind(maybeNull: 'T, f) =
            if isNull maybeNull then None else f maybeNull


module Usage =
    open MaybeBuilder
    open MaybeBuilderExtension // TODO: false positive

    let foo (s: string) =
        maybe {
            let! a = s
            return a
        }