Hello all, It seems this message was lost somehow, so i'm trying to send it again ....sorry if it comes up twice on the list ! I have this class which gives a common interface to (UniqueIndex a k) and (MultiIndex a k) : class (Show a, Key_ k) => Index_ i a k | i -> k, k -> a where buildKey :: (a -> k) insertIndex :: Id -> a -> i -> Maybe i deleteIndex :: Id -> a -> i -> i updateIndex :: Id -> a -> a -> i -> Maybe i updateIndex id oldItem newItem index = insertIndex id newItem $ deleteIndex id oldItem index Now i need to have these indexes in a list, so i declared that type : data DbIndex = forall a k i. (Show a, Key_ k, Index_ i a k) => DbIndex i Up to this point everything is fine, I can create concrete Indexes and put these different index types in the same list, after wrapping them in a DbIndex. But i don't seem to find a way to get out of this DbIndex type to actually work on the enclosed index. for instance, this doesn't work: liftDbIndex (DbIndex index) fun = DbIndex (fun index) this doesn't work either : dbIndexBuildKey (DbIndex index) value = (buildKey index) value How can i access the enclosed concrete index, via its Index_ class ? Maybe am I totally on the wrong track ? Thanks in advance, Sacha