I decided to take a look at DataKinds extension, which became available in GHC 7.4. My main concerns is that I cannot close type classes for promoted data types. Even if I fix type class argument to a promoted type, the use of encoding function still requires specification of context. I consider this an omission of potentially very useful feature. Example is below. ----------------------------------------------------------------------------------------- {-# LANGUAGE TypeOperators, DataKinds, TemplateHaskell, TypeFamilies, UndecidableInstances #-} {-# LANGUAGE GADTs #-} -- a binary numbers. infixl 5 :* data D = D0 | D1 | D :* D deriving Show -- encoding for them. data EncD :: D -> * where EncD0 :: EncD D0 EncD1 :: EncD D1 EncDStar :: EncD (a :: D) -> EncD (b :: D) -> EncD (a :* b) -- decode of values. fromD :: D -> Int fromD D0 = 0 fromD D1 = 1 fromD (d :* d0) = fromD d * 2 + fromD d0 -- decode of encoded values. fromEncD :: EncD d -> Int fromEncD EncD0 = 0 fromEncD EncD1 = 1 fromEncD (EncDStar a b) = fromEncD a * 2 + fromEncD b -- constructing encoded values from type. -- I've closed possible kinds for class parameter (and GHC successfully compiles it). -- I fully expect an error if I will try to apply mkD to some type that is not D. -- (and, actually, GHC goes great lengths to prevent me from doing that) -- By extension of argument I expect GHC to stop requiring context with MkD a where -- I use mkD "constant function" and it is proven that a :: D. class MkD (a :: D) where mkD :: EncD a instance MkD D0 where mkD = EncD0 instance MkD D1 where mkD = EncD1 -- But I cannot omit context here... instance (MkD a, MkD b) => MkD (a :* b) where mkD = EncDStar mkD mkD data BV (size :: D) where BV :: EncD size -> Integer -> BV size bvSize :: BV (size :: D) -> Int bvSize (BV size _) = fromEncD size -- ...and here. -- This is bad, because this context will arise in other places, some of which -- are autogenerated and context for them is incomprehensible to human -- reader. -- (they are autogenerated precisely because of that - it is tedious and error prone -- to satisfy type checker.) fromIntgr :: Integer -> BV (size :: D) -- doesn't work, but desired. -- fromIntgr :: MkD size => Integer -> BV (size :: D) -- does work, but is not that useful. fromIntgr int = BV mkD int -----------------------------------------------------------------------------------------