[Haskell-cafe] Inferred type less polymorphic than expected

[Joel Reymont]

Folks,

How do I fix this?

data Prop = forall a b.(Eq a, Show a) => Attr a := a

data Attr a = Attr String
     (a -> Dynamic, Dynamic -> Maybe a)
     (PU a)

type Props = M.Map String (Int, Prop)

instance Ord (Int, Prop) where
     compare (a, _) (b, _)
         | a == b = EQ
         | a > b = GT
         | otherwise = LT

makeAttr :: Typeable a => String -> PU a -> Attr a
makeAttr name pickler = Attr name (toDyn, fromDynamic) pickler
...
props :: Props -> PU Props
props m = props' $ sort $ M.toList m
     where props' [] = lift []
           props' ((_, (Attr _ _ pp := _)):xs) =
               wrap (\(a, b) -> a : b,
                     \(a : b) -> (a, b))
                        (pair pp (props' xs))


./Script/Prop.hs:80:10:
     Inferred type is less polymorphic than expected
       Quantified type variable `a' is mentioned in the environment:
         props' :: [(a1, Prop)] -> PU [a] (bound at ./Script/Prop.hs: 
79:10)
     When checking an existential match that binds
         $dEq :: {Eq a}
         $dShow :: {Show a}
         pp :: PU a
     The pattern(s) have type(s): [(a1, Prop)]
     The body has type: PU [a]
     In the definition of `props'':
         props' ((_, (Attr _ _ pp := _)) : xs)
                  = wrap (\ (a, b) -> a : b, \ (a : b) -> (a, b))  
(pair pp (props' xs))
     In the definition of `props':
         props m = props' $ (sort $ (Data.Map.toList m))
                 where
                     props' [] = lift []
                     props' ((_, (Attr _ _ pp := _)) : xs)
                              = wrap (\ (a, b) -> a : b, \ (a : b) ->  
(a, b)) (pair pp (props' xs))

	Thanks, Joel

--
http://wagerlabs.com/