GMaps -- families of map implementations indexed by the key type -- are an example on the wiki:   http://www.haskell.org/haskellwiki/GHC/Type_families I've been using something like this myself.  It sure would be nice to have a fully developed version on Hackage, and I may try to submit this myself if no one else has already done it.  But I'm running into a problem in achieving a particular optimization that I want -- namely, small tuple keys packed into Ints. For example, with a key type (Int16,Int16) it's possible to pack the key into a plain Int and use Data.IntMap. In fact, in the rough/partial implementation here (http://people.csail.mit.edu/newton/gmap/GMap.hs) I have a class for things that fit in a word: class FitInWord v where   toWord   :: v -> Word   fromWord :: Word -> v (Or fitInInt for that matter.) Which includes things like small tuples: instance FitInWord (Int16,Int16) where toWord (a,b) = shiftL (fromIntegral a) 16 + (fromIntegral b) fromWord n = (fromIntegral$ shiftR n 16, fromIntegral$ n .&. 0xFFFF) (If you know a better idea than using templates/program generation to generate every combination of small scalar tuples that fit in a word.... let me know.) What I would next *like* to do is something like the following: import qualified Data.IntMap as DI instance FitInWord t => GMapKey t where  data GMap t v           = GMapInt (DI.IntMap v) deriving Show  empty                   = GMapInt DI.empty  lookup k    (GMapInt m) = DI.lookup (wordToInt$ toWord k) m  insert k v  (GMapInt m) = GMapInt (DI.insert (wordToInt$ toWord k) v m)  alter  fn k (GMapInt m) = GMapInt (DI.alter fn (wordToInt$ toWord k) m)  toList      (GMapInt m) = map (\ (i,v) -> (fromWord$ intToWord i, v)) $    DI.toList m The problem is that there's already a more general instance of GMapKey that handles pairs by representing them as nested GMaps: instance (GMapKey a, GMapKey b) => GMapKey (a, b) where data GMap (a, b) v = GMapPair (GMap a (GMap b v)) .... Ideally, I want both of these to coexist (and to prioritize the more specific one). With normal type classes, OverlappingInstances can handle this, but with type families I get an error like the following: Conflicting family instance declarations: data instance GMap t v -- Defined at Intel/GMap.hs:108:7-10 data instance GMap (a, b) v -- Defined at Intel/GMap.hs:225:7-10 Any fixes? Thanks, -Ryan