#2850: GeneralizedNewtypeDeriving + TypeFamilies doesn't work -------------------------------------------------+------------------------- Reporter: ajd | Owner: Type: feature request | Status: Priority: normal | closed Component: Compiler | Milestone: 6.12 Resolution: fixed | branch Operating System: Unknown/Multiple | Version: Type of failure: None/Unknown | 6.10.1 Test Case: | Keywords: indexed_types/should_compile/T2850 | Architecture: Blocking: | Unknown/Multiple | Difficulty: | Unknown | Blocked By: | Related Tickets: -------------------------------------------------+------------------------- Comment (by goldfire): Almost, but not quite, due to the lack of informative roles on data/newtype instance representation tycons. I do think the fix for #8548 would fix the example in this ticket, but not something like {{{ data family D a data instance D (Maybe a) = MkDMaybe a class C (D (Maybe Int)) where ... newtype instance D [a] = MkDList (D (Maybe a)) deriving instance C (D [Age]) }}} That's all rather confusing, but I ''think'' it should work. And it would require informative roles on data/newtype instance representation tycons. These roles are currently all nominal. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/2850#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler