[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):

 Edward and I discussed this.  What if we wrote this:
 {{{
 pattern VCons :: a -> Vec a n -> Vec a (n + 1)
 pattern VCons x xs = Vec (x : unVec xs)
 }}}
 What makes it OK to use bidirectional "=" is that the RHS is invertible.
 Constructors are invertible. '''But so is `unVec`''', because it's inverse
 is just `Vec`.

 So maybe we should just expand the class of invertible RHSs a little?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12203#comment:3>
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