[Haskell-cafe] Ambiguous type signature in class declaration

[Bo Herlin]

Hi

I am trying to make a class like this:

 > 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
 >   count :: a -> Maybe Integer -- Nothing means infinity

with possible instances like

 > data DPrime = Prime deriving (Show)
 >
 > instance CRank DPrime Integer where
 >   rank Prime x = Nothing -- to be implemented: (rank Prime 11) should 
give (Just 4)
 >   unrank Prime r = Nothing -- to be implemented: (unrank Prime 4) 
should give (Just 11)
 >   count _ = Nothing -- Nothing means infinity

and

 > data DFibonacci = Fibonacci deriving (Show)
 >
 > instance CRank DFibonacci Integer where
 >   rank Fibonacci x = Nothing -- to be implemented
 >   unrank Fibonacci r = Nothing -- to be implemented
 >   count _ = Nothing -- Nothing means infinity

but all i get is

ERROR "./Cafe.lhs":8 - Ambiguous type signature in class declaration
*** ambiguous type : CRank a b => a -> Maybe Integer
*** assigned to    : count

Any suggestions anyone?

Thanks in advance
/Bo