Re: [Haskell-cafe] Ambiguous type signature in class declaration

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?