[GHC] #12924: Pattern Synonyms in Typeclasses

4 Dec 2016

      #12924: Pattern Synonyms in Typeclasses
-------------------------------------+-------------------------------------
           Reporter:  xcmw           |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:  pattern        |  Operating System:  Unknown/Multiple
  synonym type class                 |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 data A t where
    AX :: A X
    AY :: A Y
    AZ :: A Z

 class HasX a where
      data pattern X :: a X

 instance HasX A where
     data pattern X = AX

 d :: A t -> String
 d = \case
     X  -> ""
     AY -> ""
     AZ -> ""
 }}}

 The above example should compile and emit no warnings with -fwarn-
 incomplete-patterns. Would require
 https://ghc.haskell.org/trac/ghc/ticket/8779

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12924
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