Joel Reymont wrote:
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))
This doesn't give you a (PU Props), but a (PU [exists a . a]) or something, which is bogus syntax, since the idea is already nonsensical. You have to dismantle and create 'Prop's if you want to put them into a list, and you forgot a M.fromList somewhere, too. You probably want something like this (untested, in order not to spoil the fun for you):
props :: Props -> PU Props props m = wrap M.toAscList M.fromAscList props' where props' [] = lift [] props' ((key, (Attr str casts pp := val)):xs) = wrap (\_ -> val : xs, \(val' : xs) -> ((key, Attr str casts pp := val'), b)) (pair pp (props' xs))
If you create the pickler from one concrete instance of Props, then use it to (un-)pickle another, you will get bogus results and propably a pattern match failure. Basically you have thrown all static type checking out the window when you create the Attr type, and it will come back to haunt you. You probably also want to ask yourself if you need this heavy machinery or if putting the properties into records is the more sensible thing to do. Udo.