[GHC] #12203: Allow constructors on LHS of (implicit) bidirectional pattern synonym

[GHC]

#12203: Allow constructors on LHS of (implicit) bidirectional pattern synonym
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        Reporter:  ezyang            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:                    |  PatternSynonyms
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 mpickering):

 It seems like quite a big change to me to specify the expression rather
 than the pattern. It also raises questions in the case of explicitly
 bidirectional pattern synonyms, do you specify two expressions? One which
 gets actually converted to a pattern  and one which actually gets used in
 expressions? What about for implicitly bidirectional pattern synonyms do
 we still specify an expression on the RHS?

 Still quite tricky with some details to be worked out!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12203#comment:9>
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