[GHC] #14112: bang patterns on pattern synonyms? (left vs right hand sides)

[GHC]

#14112: bang patterns on pattern synonyms? (left vs right hand sides)
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        Reporter:  carter            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
                                     |  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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Changes (by RyanGlScott):

 * keywords:   => PatternSynonyms


Comment:

 I'm not surprised that putting bang patterns on the left-hand side of a
 pattern synonyms doesn't work. After all, the left-hand side simply binds
 pattern variables. No more, no less.

 I am surprised, however, that putting bang patterns on the //right//-hand
 side does work. Especially since the right-hand sides of simply
 bidirectional pattern synonyms are supposed to be valid expressions, and
 `JustC !a` certainly doesn't feel like one.

 Curiously, the users' guide documentation is quite silent about the
 relationship between simply bidirectional pattern synonyms and bang
 patterns. To fix this quandary, we could either:

 1. Disallow bang patterns entirely on the right-hand sides of simply
 bidirectional pattern synonyms (like we do with wildcard patterns and view
 patterns).
 2. Document the current behavior as expected. `JustC !a` is bit strange-
 looking from an expression standpoint, but at the same time, it does work
 and accomplishes a useful task, so perhaps we shouldn't throw the baby out
 with the bathwater.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14112#comment:1>
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