[GHC] #16311: Suggest -XExistentialQuantification for 'forall' in data declarations

[GHC]

#16311: Suggest -XExistentialQuantification for 'forall' in data declarations
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        Reporter:  int-index         |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
  (Parser)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by int-index):

 > Do we need both? Or is there a way we could consolidate the two somehow?
 Would the answer change if we made forall permanently a keyword in types?

 No, we don't need both, and the `forall` proposal makes the matters
 trivial. See https://gitlab.haskell.org/ghc/ghc/merge_requests/363

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16311#comment:3>
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