[GHC] #12087: Inconsistency in GADTs?

[GHC]

#12087: Inconsistency in GADTs?
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        Reporter:  Iceland_jack      |                Owner:  RyanGlScott
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  GADTs
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11540            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 The underlying issue here -- that the type of GADT constructors is a bit
 inflexible -- is important in the context of Dependent Haskell. For
 example, I might want to say

 {{{#!hs
 data Foo a where
   MkFoo :: pi (x :: Type) -> (F x ~ Bool) => Foo x
 }}}

 This takes a ''visible'' type `x` and then asserts some constraint on `x`.
 The constraint must come ''after'' binding `x`, of course! So loosening up
 these rules will actually increase expressiveness once we have dependent
 types.

 Re comment:5, recall that `:type` takes an ''expression'', upon which it
 performs type inference. Thus it stands to reason that quantifiers, etc.,
 will get shuffled around. This is why `:type +v` got added, so that the
 type is just reported as is, without any instantiation/regeneralization.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12087#comment:6>
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