Hi again Next approach:
module Cafe where
class CRankable a where rank :: a b -> b -> Maybe Integer -- Nothing means b is out of range or badly constructed unrank :: a b -> Integer -> Maybe b -- Nothing means rank is out of range count :: a b -> Maybe Integer -- Nothing means infinity
This works ok:
data CPrime a = Prime deriving (Show)
instance CRankable CPrime where rank Prime x = Nothing unrank Prime r = Nothing count Prime = Nothing
But once i start returning anything but Nothing like:
data CPrime' a = Prime' deriving (Show)
instance CRankable CPrime' where rank Prime' x = Just x unrank Prime' r = Just r count Prime' = Nothing
i get: ERROR "./Cafe.lhs":26 - Inferred type is not general enough *** Expression : rank *** Expected type : CRankable CPrime' => CPrime' a -> a -> Maybe Integer *** Inferred type : CRankable CPrime' => CPrime' Integer -> Integer -> Maybe Integer so where exactly do i say a is an Integer? /Bo