This is the latest varient. Let me know whether you guys find this interesting, as I am coding what I need for a practical application, it may be tedious for you... I am still of the opinion that some kind of polygenious list (or maybe heteromorphic?) would be good in the standard libraries, whether it my code, someone elses or some combination. My initial feeling would be to standardise our function , class and type naming so the different kinds of list/set can be put into the same codebase, possibly aimed at being Data.PolyLists, also what is required to get these in the library - just the code, neat code, a report, or some kind of proposal ? infixr 1 `MCons` data MNil = MNil deriving Show data MCons l a r = a `MCons` r deriving Show class MRelation r instance MRelation MNil instance MRelation r => MRelation (MCons l a r) This is a labelled list. It uses a phantom type to label each node, but it theoretically needs no storage for the label... Standard list functions operate as before, and ignore the phantom type. class MRelation r => MList r where mHead :: MCons l a r -> a mTail :: MCons l a r -> r mIsEmpty :: r -> Bool instance MList MNil where mHead (x `MCons` _) = x mTail (_ `MCons` _) = MNil mIsEmpty MNil = True instance MList r => MList (MCons l a r) where mHead (x `MCons` _) = x mTail (_ `MCons` xs) = xs mIsEmpty (_ `MCons` _) = False I have the usual suspects fold/map/zip/unzip, and natural number lookup coded. This is the nice touch though, an indexed lookup on the phatom type, which finds the Nth occurance of a given type in the list: class (MRelation r,Nat n) => MLookup l n r a | l n r -> a where mLookup :: r -> l -> n -> a instance MRelation r => MLookup l Zero (MCons l a r) a where mLookup (x `MCons` _) _ Zero = x instance MLookup l n r b => MLookup l (Suc n) (MCons l a r) b where mLookup (_ `MCons` xs) l (Suc n) = mLookup xs l n instance MLookup l n r b => MLookup l n (MCons m a r) b where mLookup (_ `MCons` xs) l n = mLookup xs l n heres an example: data Name = Name data Size = Size data Weight = Weight test = MCons Name String (MCons Size Int (MCons Weight Float MNil)) test = "Box" `MCons` 3 `MCons` 1.1 putStrLn $ show $ mLookup test Name zero putStrLn $ show $ mLookup test Size zero putStrLn $ show $ mLookup test Weight zero putStrLn $ show $ mLookup (test `mProduct` test) Name one putStrLn $ show $ mLookup (test `mProduct` test) Weight zero These last two show how to access the labels from the right and left sides of the product when some/all domains have the same name. Regards, Keean.