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