[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):

 Good point. I think we may require parens around or patterns. So the
 parsing rule would be like:

 {{{
 pat ::= '(' or-pattern-or-pat ')'
      |  ... (where none of the productions accept '|')

 or-pattern-or-pat
     ::= pat '|' or-pattern-or-pat
      |  pat
 }}}

 I personally dislike messier syntax like `:|` and `:|:`, because I'm
 hoping to use this syntax instead of `_` patterns, and I may need to chain
 a lot of constructors.


 {{{
 f (T1 ...) = ...
 f (T2{} | T3{} | T4{}) = ...
 }}}

 vs

 {{{
 f (T1 ...) = ...
 f (T2{} :|: T3{} :|: T4{}) = ...
 }}}

 Second one looks really bad IMHO. In addition, `:|:` is a valid operator
 syntax, but `|` is a reserved operator.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/3919#comment:18>
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