#11732: Deriving Generic1 interacts poorly with TypeInType -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: TypeInType, | Generics Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): No instantiation check. But what GHC does right now is, for every class in a datatype's `deriving` clause, drop as many type parameters as there are arguments to the class being derived. For example {{{ data Foo a deriving (Show, Functor) }}} GHC knows that we mean `instance Show (Foo a)` and `instance Functor Foo`. But {{{ data Proxy k (a :: k) deriving Functor }}} would try `instance Functor (Proxy k)`, which is ill-kinded. GHC is surely clever enough to figure out that it should really do `instance Functor (Proxy *)`, but I think it should refrain from doing this. More to the point of this ticket: I have a bad feeling my fix for #11357 is utterly broken, in that it uses the `tc_args` from the instance tycon despite using visibilities from the parent tycon. Of course, the `tc_args` don't match up. I suppose a `mkFamilyTyConApp` would work nicely here. But you seem to know much more about generics in GHC than I do. Do you think you could make this fix? I really do think we just need to use `mkFamilyTyConApp` in the right spot here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11732#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler