#10524: PolyKinds doesn't interact well with DeriveFunctor -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by rwbarton): Good question. In principle there are classes at other kinds `Functor1`, `Functor2` (not part of a systematic naming scheme) for which you can write {{{ instance (Functor1 (f :: (* -> *) -> *), Functor2 (g :: * -> (* -> *))) => Functor (Compose f g) }}} but GHC will probably never be able to derive that instance. (Maybe if eventually get a polykinded `Functor`, but then the deriving clause could produce a kind-polymorphic instance and there is no problem.) So, I don't see any real issue with having ordinary `deriving` producing an instance for `Functor (Compose * f g)`, to write the kind variable explicitly. However, it's certainly more clear-cut with the standalone deriving declaration, since then the kind variable is determined by the context which you wrote explicitly. I think GHC may have some general principles regarding ordinary deriving declarations and how they are less general than standalone deriving, but I never understood the exact details (aside from the fact that a standalone deriving declaration lets you specify the context). Maybe they don't have anything to say about this case with a kind variable anyways. Not sure where that leaves this ticket; the behavior of HEAD is a bug that I'll file separately. Maybe a feature request dependent on the resolution of that bug. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10524#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler