[GHC] #8779: Exhaustiveness checks for pattern synonyms

[GHC]

#8779: Exhaustiveness checks for pattern synonyms
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        Reporter:  nomeata           |                Owner:  mpickering
            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):  Phab:D2669
       Wiki Page:                    |
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Comment (by mpickering):

 To respond to an earlier point that Reid and others have made. The
 potential problems with this pragma only highlight design compromises with
 pattern synonyms. The specific issue being that pattern synonyms are ad-
 hoc collections rather than a complete set of covering patterns.

 Reid's suggestion of improving exhaustiveness checking by seeing if the
 view function is injective is akin to suggesting that one should only be
 allowed to define complete sets of pattern synonyms in the first place.

 I see the design I have implemented as the sensible way of doing so. If we
 forced all users to write pattern synonyms in the way that Reid described
 then we would have been better off implementing views!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8779#comment:45>
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