[GHC] #8581: Add support for explicitly-bidirectional pattern synonyms

[GHC]

#8581: Add support for explicitly-bidirectional pattern synonyms
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        Reporter:  cactus            |            Owner:  cactus
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:  5144
        Blocking:                    |  Related Tickets:
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Comment (by Iceland_jack):

 This would be very nice to have, +1.

 The possibility of `_` being a shorthand for `undefined` is intriguing. It
 allows `_` to be a run-of-the-mill pattern exported by `Prelude` that may
 be redefined by users! The regular wildcard meaning where the expression
 is `undefined` could be defined as such with the proposed syntax:

 {{{
 pattern _ <- a where
   _ = undefined

 failure :: a -> b
 failure _ = _
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8581#comment:2>
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