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