[Haskell-cafe] Ambiguous type signature in class declaration

[Bo Herlin]

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