#9112: data families with representational matching -------------------------------------+------------------------------------ Reporter: jwlato | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by simonpj): The trouble is that you can say {{{ data instance MVector s Int = ...rep1... data instance MVector s Age = ...rep2... }}} so that `MVector` over `Int` has an entirely different representation to `MVector` over `Age`. Indeed that is often the very reason that people define a newtype in the first place! For example, if you want `sort` to sort into reverse order, can write {{{ import Data.Ord( Down(..) ) downSort :: Ord a => [a] -> [a] downSort xs = coerce (sort (coerce xs :: [Down a])) }}} We coerce the `[a]` to `[Down a]`, then sort (using `Down`'s ordering), then coerce back. It's going to be quite confusing `data instance` can sometimes match on a newtype, and sometimes not. And then there are nested cases to worry about: {{{ newtype MVector s [Age] = ... newtype MVector s [Int] = ... }}} Moreover, you still (presumably) want `MVector s Age` and `MVector s Int` to be distinct types! None of this smells good to me. But here's an idea. You want `MVector s Age` and `MVector s Int` But you want them to be represented the same way. That's what we use newtypes for. So how about this: {{{ newtype instance MVector s Age = MVA (MVector s Int) }}} Would that help? For example, this compiles without complaint: {{{ {-# LANGUAGE MultiParamTypeClasses, TypeFamilies, GeneralizedNewtypeDeriving #-} module T9112 where class MVectorClass (v :: * -> * -> *) a where basicLength :: v s a -> Int data family MVector s a data instance MVector s Int = MV -- implementation not important newtype Age = Age Int deriving (MVectorClass MVector) newtype instance MVector s Age = MV1 (MVector s Int) instance MVectorClass MVector Int where basicLength x = 0 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9112#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler