> would it be a solution if you defined
>
> > data RetVal b a = Result a | Exception b
>
> in this case you could say
>
> > instance Monad (RetVal b) where
> > return = Result
> > Result x >>= f = f x
> > Exception x >>= f = Exception x
Incidentally, the predefined data type `Either' may be used for this
purpose.
> instance Monad (Either a) where
> Left a >>= k = Left a
> Right a >>= k = k a
> return = Right
>
> raise :: a -> Either a b
> raise s = Left s
`Right' means ok ;-).
Cheers, Ralf
PS: Hallo Peter.
