Erm... okay my mistake thats not the problem... although it is to with state... When decaring a monad the return type is included in the class definition (it is a constructor class) This means the return value MUST be the last parameter to the type, so you need: type ReverseType a string = (string ->(string,a)) data Reverse string a = Reverse (ReverseType a string) instance Monad (Reverse s) where return x = Reverse (\text -> (text,x)) (Reverse p) >>= k = Reverse p3 where p3 s0 = p2 s1 where (Reverse p2) = k a (s1,a)=p s0 I was confused by the unusual format for bind... I would wtite this: instance Monad (Reverse s) where return x = Reverse $ \s -> (s,x) (Reverse p) >>= k = Reverse $ \s -> case p s of (s',a) -> (\(Reverse r) -> r) (k a) s' or even better define the data type as: data Reverse string a = Reverse { runReverse :: ReverseType a string } instance Monad (Reverse s) where return x = Reverse $ \s -> (s,x) (Reverse p) >>= k = Reverse $ \s -> case p s of (s',a) -> runReverse (k a) s' Regards, Keean.