[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 dfeuer):

 Replying to [comment:45 goldfire]:
 > The idea in comment:39 has grown on me. So let me turn this into a
 concrete proposal:
 >
 > 1. Relax Haskell's current restriction around capitalized identifiers.
 That is, any old variable can now begin with a capitalized letter or a
 colon. Capitalized variables can be defined only by function-definition
 syntax, never by patterns. That is, `Foo = 5` is OK, as is `Bar x = x +
 2`. On the other hand, `Just Quux = listToMaybe blurgh` would not be OK.

 I like the general theme very much, but I think we use the `constructor`
 keyword to introduce capitalized identifiers as well as to export them
 unbundled. This distinguishes them syntactically from pattern bindings and
 makes it immediately obvious that something strange is happening. I agree
 that we don't need to allow them to be defined using pattern bindings;
 that would complicate things with little benefit.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8581#comment:46>
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