[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 simonpj):

 Yes, I think it would have to be the latter.

 Several open issues remain:

  * We need pattern signatures. The workaround (of a signature embedded in
 the pattern) doesn't work too well for these more complicated types.

  * In fact having signatures embedded in a pattern synonym is problematic,
 because signatures behave differently in patterns and in terms.
 {{{
 g x = Just x :: Maybe a
 f (Just x :: Maybe a) = x
 }}}
   In `g`, the type signature is implicitly quantified, so it really means
 `Just x :: forall a. Maybe a`, and the definition will therefore be
 rejected.  But in the definition of `f`, the 'a' in the pattern is a
 ''binding'' occurrence, that scopes over the RHS; there is no implicit
 quantification.

   I'm inclined simply to disallow signature in the RHS of a pattern
 synonym.

  * We have no good syntax for the required/provided issue. I thought this
 was written up on the [wiki:PatternSynonyms Pattern Synonym wiki page] but
 it isn't.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8968#comment:18>
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