[GHC] #8779: Exhaustiveness checks for pattern synonyms

[GHC]

#8779: Exhaustiveness checks for pattern synonyms
-------------------------------------+-------------------------------------
        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 rwbarton):

 I feel like my suggestion, whatever it was, must have been
 misinterpreted... I'm concerned about situations in which there are no
 view patterns, at all.

 Take this scenario. I have a data type with a lot of `Either` fields
 {{{#!hs
 data T = T String (Either Int T) (Either Int T) (Either Int T) (Either Int
 T)
 }}}
 and I need to write a bunch of functions like
 {{{#!hs
 f (T s (Left _) (Left _) (Left n) (Right _)) = replicate n s
 f (T s (Left _) (Left n) (Right _) (Right _)) = replicate (n-1) s
 -- ...
 }}}
 It sure would be handy if I could define
 {{{#!hs
 pattern L <- Left _
 pattern R <- Right _
 }}}
 to condense the long left-hand sides. It just seems so obvious that I
 should be able to use a ''pattern synonym'' as a shorthand for a pattern,
 without having to go through extra contortions to appease the
 exhaustiveness checker.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8779#comment:47>
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