[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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Comment (by RyanGlScott):

 Actually, I want to take back my proposal in comment:3. My idea relies on
 the ability to put "bang patterns" on variable binders, which is something
 that simply isn't possible anywhere else. (We could change the pattern
 variable binders to be proper patterns and only allow them to be variables
 or variables adorned with bangs, but that would be an exceedingly strange
 design.)

 In light of this, I would suggest that we endorse explicitly bidirectional
 pattern synonyms as the only way to achieve a "two-way" strict pattern
 synonym. That is:

 {{{#!hs
 pattern MyPair1 x y <- MkPair !x !y where
   MyPair1 !x !y = MkPair x y
 }}}

 Therefore, the fact that you can currently do this:

 {{{#!hs
 pattern MyJust a = JustC !a
 }}}

 Should be treated as a bug. `JustC !a` makes no sense as an expression,
 and moreover, it doesn't even give the strictness behavior that you'd
 expect, as discovered in comment:2.

 Does that sound agreeable, carter?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14112#comment:4>
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