#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): Replying to [https://ghc.haskell.org/trac/ghc/ticket/11357#comment:13 goldfire]:
I'm frankly unsure (without more research) as to what the desired behavior should be here.
The reason for this restriction dates back to #5939, when someone wanted to try to standalone derive this: {{{#!hs data T a = T a deriving instance Generic (T Bool) }}} This seems intuitive, but [https://ghc.haskell.org/trac/ghc/ticket/5939#comment:1 Pedro couldn't find a way to make it work], since the generated `Rep` type only uses information from `T`'s `TyCon`, so there was no good way to relate the `TyVar` `a` with `Bool`. He then declared that [https://ghc.haskell.org/trac/ghc/ticket/5939#comment:2 such instances should be rejected altogether], hence this instantiation check. Whether the instantiation check is a good idea (and ways to get around it) are a side issue, I feel like. The real issue is that data family instances classify their types as visible/invisible in a completely different manner than vanilla datatypes. I did a bit of digging into the `ProxyFam` example above. With this: {{{#!hs data Proxy k (a :: k) = ProxyCon deriving Generic1 }}} [http://git.haskell.org/ghc.git/blob/c37a583fb9059053f83f1ab0c3cb7eb7047b1a31... partitionInvisibles user_tc id tc_args] yields `([],[*])`, like you'd expect (where `k` has been instantiated to `*` due to `Generic1` being of kind `(* -> *) -> Constraint`). But with this: {{{#!hs data family ProxyFam (a :: y) (b :: z) data instance ProxyFam k (a :: k) = ProxyFamCon deriving Generic1 }}} [http://git.haskell.org/ghc.git/blob/c37a583fb9059053f83f1ab0c3cb7eb7047b1a31... partitionInvisibles user_tc id tc_args] yields `([*],[])`! Somehow, `k` is being treated as invisible when it clearly shouldn't. It's beyond my pay grade to speculate on //why// that is, but it's definitely [https://ghc.haskell.org/trac/ghc/ticket/11376#comment:12 not the only] `-XTypeInType`-related data family discrepancy. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11732#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler