[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 simonpj):

 Great or not, it's pretty simple.  Adding newtype patterns on the left is
 really quite tricky.  It took me 15 mins to grok.

 As to the parser, it parses patterns as expressions, and then converts
 them to patterns.  Then the patsyn code converst it back (for
 bidirectional).  So maybe we should leave it as an expression!  The rule
 is still simple: to use it as an expression, just use the exression.  To
 use it as a pattern, just treat `unVec e` as the view pattern `(Vec ->
 e)`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12203#comment:8>
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