#8177: Roles for type families -------------------------------------+------------------------------------ Reporter: simonpj | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 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): I agree that the `vector` issue is related to this feature. In summary, we would like the following to work (greatly simplified from vector WLOG): {{{ class MVectorClass (v :: * -> * -> *) a where basicLength :: v s a -> Int data family MVector s a data instance MVector s Int -- implementation not important newtype Age = Age Int deriving (MVectorClass MVector) -- rejected }}} The deriving is rejected because it generates {{{ basicLength = coerce (basicLength :: MVector s Int -> Int) :: MVector s Age -> Int }}} If my understanding of role inference is correct, allowing {{{ type role instance MVector nominal representational }}} would allow for this coerce to take place. This means that we can't currently derive Unbox instances for vectors of newtypes (see https://github.com/haskell/vector/issues/16), which is rather unfortunate. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8177#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler