#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 jwlato): Simon, it's possible that suggestion will work, with a few other modifications to the way roles are currently handled. It doesn't work with vector/ghc as currently released due to some other methods. For example, if I add `basicUnsafeRead` {{{ class MVectorClass (v :: * -> * -> *) a where basicLength :: v s a -> Int basicUnsafeRead :: PrimMonad m => v (PrimState m) a -> Int -> m a }}} It's not possible to coerce `basicUnsafeRead` because of the coercion from `m Int` to `m Age`. There are a number of other methods that also can't be coerced, but I think they're all because `m`s type parameter is at a nominal role. If it were possible to express that `m`s parameter need only be representationally equal, I think this would work. (maybe related to the current issues with deriving and AMP?) I'm not sure what's involved with making that coercion happen, but it seems decidely less tricky than representationally-equal data families (I have quite a bit more to say about that also, but I'll leave it for another venue). I'm changing the title of this ticket to more closely reflect the actual goal, e.g. deriving vector instances. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9112#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler