[Haskell-cafe] Ambiguous type signature in class declaration

[Bo Herlin]

Doh, i have another question:

Lets say i do as you wrote:

 > class CRank a b where
 >   rank :: a -> b -> Maybe Integer -- Nothing means b is out of range 
or badly constructed
 >   unrank :: a -> Integer -> Maybe b -- Nothing means rank is out of range
 >
 > class CCountable a where
 >   count :: a -> Maybe Integer -- Nothing means infinity

how do i make a test-class like this:

 > class (CRank a b,CCountable a) => CTestable a where
 >   testOne :: a -> Integer -> Bool
 >   testUpTo :: a -> Integer -> Bool
 >   testOne x r = ((unrank x r) >>= (rank x)) == (Just r)
 >   testUpTo x mr = foldr1 (&&) (map (testOne x) [0..m])
 >     where
 >       m = f (count x)
 >       f Nothing = mr
 >       f (Just y) = min (y-1) mr

this gives me:

ERROR "./Cafe.lhs":14 - Undefined type variable "b"

If i remove the b like "class (CRank a,CCountable a) => CTestable a 
where" i get:

ERROR "./Cafe.lhs":14 - Wrong number of arguments for class "CRank"

As a Haskell-newbee i get totally confused.

/Bo