[GHC] #10995: Existentials in newtypes

[GHC]

#10995: Existentials in newtypes
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        Reporter:  crockeea          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10715            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Ahh...

 There is clearly a bug here.  But it's a bug hiding a design question.  If
 you see this:
 {{{
 data T = MkT (forall a f. f a -> Int)
 }}}
 which of these two would you expect to get?
 {{{
 data T1 @k = MkT (forall (a::k) (f::k->*). f a -> Int )

 data T2 = MkT (forall k. forall (a::k) (f::k->*). f a -> Int)
 }}}
 That is, where should the kind be quantified?

 A similar choice arises for ordinary function type signatures; see this
 comment in `TcHsType`:
 {{{
 Note [Kind generalisation]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 We do kind generalisation only at the outer level of a type signature.
 For example, consider
   T :: forall k. k -> *
   f :: (forall a. T a -> Int) -> Int
 When kind-checking f's type signature we generalise the kind at
 the outermost level, thus:
   f1 :: forall k. (forall (a:k). T k a -> Int) -> Int  -- YES!
 and *not* at the inner forall:
   f2 :: (forall k. forall (a:k). T k a -> Int) -> Int  -- NO!
 Reason: same as for HM inference on value level declarations,
 we want to infer the most general type.  The f2 type signature
 would be *less applicable* than f1, because it requires a more
 polymorphic argument.
 }}}
 On this basis, to be consistent, we should go for `T1` not `T2`.  (Which
 raises the question of how you could get `T2` or `f2` if you wanted them.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10995#comment:2>
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