[GHC] #8779: Exhaustiveness checks for pattern synonyms

[GHC]

#8779: Exhaustiveness checks for pattern synonyms
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        Reporter:  nomeata           |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.8.1
  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 rwbarton):

 In a case like this you would define a view `data CommandView = A1 | M1;
 viewCommand :: Command -> CommandView` which is presumed to be total
 thanks to your invariant. Then define `pattern AddOne = (viewCommand ->
 A1)`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8779#comment:35>
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