[GHC] #14332: Deriving clauses can have forall types

[GHC]

#14332: Deriving clauses can have forall types
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Replying to [comment:14 simonpj]:
 > Ryan, might you be able to do this?

 Er, I doubt it. This sounds like it goes well beyond my abilities as a GHC
 hacker, since it requires intricate knowledge of:

 * Somehow informing GHC of which type variables are "OK to unify" for
 instance, the `k` in `data Proxy (a :: k)` is OK to unify, but the `k` in
 `deriving (C (a :: k))` is not—what is the secret sauce in GHC to specify
 this? I have no idea.

   Actually, it's even more subtle than that. There are scenarios when
 you'd want to unify kind variables in `deriving` types: when they're
 //invisible//. For instance:

   {{{#!hs
   newtype Identity a = Identity a deriving Generic1
   }}}

   Here, `Generic1 :: (k -> *) -> Constraint`. But note that we never write
 `k` explicitly, so it's more like `deriving (Generic1 {k})`. However, we
 ultimately unify `k` with `*` in the end, giving us `deriving (Generic1
 {*})`. Somehow, we //also// have to inform GHC that this is OK. Urp...
 * WTF this "skolem" business from comment:9 is about. (I don't know if I
 could even give a proper definition of this word, let alone profitably
 implement it.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14332#comment:15>
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