#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 RyanGlScott): Interesting, you'd be alright with the second option? I like that idea since it requires //zero// instantiation checks, but you did object to it earlier:
I claim that
{{{ data Proxy k (a :: k) = ProxyCon deriving Generic1 }}}
should fail outright. I don't think GHC should be in the business of inferring values for visible parameters like `k`, even when it could. Instead, it should be this:
{{{ deriving instance Generic1 (Proxy *) }}}
Note that this applies to `Functor` as much as it does to `Generic1`.
...
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.
-- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11732#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler