[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:  8.0.1
       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 simonpj):

 Let's not think about implementation before we have a ''design''.   I have
 not read the entire thread again, but I'm pretty convinced that

 * We can't have two different types, one for construction and one for
 pattern matching

 I think it'll just be too confusing to have two types.  It's bad enough to
 have this provided/required stuff without, in addition, having a
 completely separate type for construction.  Are you seriously proposing to
 have two signatures for each pattern synonym?  (Optionally, I assume.)

 So: if you give a pattern signature, I think it has to work for both
 construction and pattern matching. If you need extra constraints for
 construction, define a smart constructor (that can happen today with
 regular data constructors).

 Let's not over-elaborate until we have more experience.  There are plenty
 of [https://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms open tickets to
 tackle on the pattern-synonym front].

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8581#comment:34>
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