[GHC] #8581: Pattern synonym used in an expression context could have different constraints to pattern used in a pattern context

[GHC]

#8581: Pattern synonym used in an expression context could have different
constraints to pattern used in a pattern context
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        Reporter:  cactus            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:
      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 goldfire):

 Replying to [comment:41 dfeuer]:
 > [Allowing an expression-only "pattern synonym"] emphasizes the
 orthogonality of pattern synonyms and constructor synonyms; I tend to find
 orthogonal features easier to understand. The current reuse of the `where`
 keyword to add a constructor synonym is also troubling. It would feel
 cleaner to me to let these be completely separate declarations.

 OK. I still don't want this feature, but I understand your reason.

 >
 > BTW, what ever happened to the idea of letting a module re-export a type
 and associate pattern synonyms with it? I lost track of that and haven't
 had a chance to upgrade to 8.0 yet.

 This is implemented, as described (very briefly) in the second half of
 [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html
 #import-and-export-of-pattern-synonyms this section] of the user manual.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8581#comment:42>
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