[GHC] #14004: G

[GHC]

#14004: G
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        Reporter:  zaoqi             |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      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 zaoqi):

 Replying to [comment:11 AntC]:
 > Replying to [comment:10 zaoqi]:
 >
 > Good. You've got it to compile. So we can close the ticket.
 >
 > I apologise: my comment:9 is wrong. I must have confused myself playing
 with it. Ben is right:
 >
 > > ... it's not even possible to know at runtime whether the bound `a`
 and `b` values are of the same type.
 >
 > In fact variables `a`, `b` bound by GADT `Dyn` are "rigid type
 variables". GHC always treats them as different types. They're
 existentially quantified. See
 [http://downloads.haskell.org/~ghc/8.0.2/docs/html/users_guide/glasgow_exts.html
 #generalised-algebraic-data-types-gadts The User Guide on GADTs]
 >
 > So `instance Eq Dyn` will always return False.

 How to write the correct `instance Eq Dyn`, do not change `data Dyn`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14004#comment:12>
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