[GHC] #3919: Or-patterns as GHC extension

[GHC]

#3919: Or-patterns as GHC extension
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        Reporter:  BjornEdstrom      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  ⊥
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
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 osa1):

 If we could come up with a nice syntax for function definitions with or-
 patterns this would be really useful for GHC's own code too.

 Often we have to re-compile GHC many times and test on some programs to
 discover all the places we need to update after adding a new data
 constructor, because of "catch-all" code like this:

 {{{#!haskell
 f A = ...
 f B = ...
 f unexpected = pprTrace "f" (ppr unexpected)
 }}}

 With or patterns, we could do this instead:

 {{{#!haskell
 f A = ...
 f B = ...
 f unexpected@(C{} | D{} | E{}) = pprTrace "f" (ppr unexpected)
 }}}

 (just made up a syntax)

 This would be very convenient, because now I get a warning after adding
 data constructor D - instead of discovering this code after a runtime
 failure. And what happens if I couldn't discover this code because I
 couldn't come up with an example program that makes GHC run this function?
 So it's a bit tricky and annoying right now, and it's costing us time
 because of recompilations and tests.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/3919#comment:11>
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