[GHC] #8968: Pattern synonyms and GADTs

[GHC]

#8968: Pattern synonyms and GADTs
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        Reporter:  kosmikus                   |            Owner:
            Type:  bug                        |           Status:  new
        Priority:  normal                     |        Milestone:
       Component:  Compiler (Type checker)    |          Version:
      Resolution:                             |  7.8.1-rc2
Operating System:  Unknown/Multiple           |         Keywords:
 Type of failure:  GHC rejects valid program  |     Architecture:
       Test Case:                             |  Unknown/Multiple
        Blocking:                             |       Difficulty:  Unknown
                                              |       Blocked By:
                                              |  Related Tickets:
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Comment (by kosmikus):

 I agree completely with the function example. I'm just sure about whether
 the pattern synonym situation is really analogous. The reason is that in
 {{{
 data T a b where
   MkT :: a -> T a Bool
   MkX :: T a b

 pattern P = MkT True
 }}}
 I could just pretend that `P` is another constructor of `T`:
 {{{
 data T a b where
   MkT :: a -> T a Bool
   MkX :: T a b
   P   :: T Bool Bool
 }}}
 Then both `f` and `g` as shown above using `P` would type-check. So if for
 `P` as a constructor, the information that the "expression type" is `T
 Bool Bool` and the "target type" is `T a b` is sufficient, then why can't
 we do the same for `P` as a pattern synonym. That's still modular, IMHO.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8968#comment:12>
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