[Haskell-cafe] Ambiguous type signature in class declaration

[robert dockins]

See:

http://haskell.org/hawiki/FunDeps

>  > 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
> 

[snip]

> 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?
>