#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 [comment:2 goldfire]:
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.
Agreed.
Instead, it should be this:
{{{ deriving instance Generic1 (Proxy *) }}}
I'm OK with this idea, but we'd need to be able to teach `gen_Generic_binds` about substituting type variables for user-supplied types somehow, since they are reflected in the generated `Rep`/`Rep1` associated type instance. I have no idea how challenging that would be to implement.
Note that this applies to `Functor` as much as it does to `Generic1`.
So you'd propose applying an instantiation check to every class in a `deriving` clause, then?
As for the `partitionInvisibles` piece: yes, that is all suspicious. I conjecture that we should never call `partitionInvisibles` on a data ''instance'' tycon, but only on the data ''family'' tycon.
That's what we're doing at the moment, no? We're calling `filterOutInvisibleTypes` in `canDoGenerics` [http://git.haskell.org/ghc.git/blob/c37a583fb9059053f83f1ab0c3cb7eb7047b1a31... here] (which indirectly invokes `partitionInvisibles`), and your [https://git.haskell.org/ghc.git/commitdiff/1eefedf7371778d1721d9af9247c2eff1... earlier patch] changed the `TyCon` being passed to `filterOutInvisibleTypes` to the data family tycon. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11732#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler