[GHC] #10756: Allow users to indicate inaccessible patterns

[GHC]

#10756: Allow users to indicate inaccessible patterns
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        Reporter:  edsko             |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by YitzGale):

 Replying to [comment:9 goldfire]:

 Correct. The first two would be valid and retain their current meaning.
 The third is invalid for two reasons: because `Just` requires an argument,
 and because the bound RHS value is omitted after an impossible pattern.

 I'm not sure what you mean by "top level". My proposal is similar to many
 other features of Haskell as well as almost every other language - that
 certain tokens are given special meaning in specific syntactic contexts.
 Why is that a weird wrinkle? A few existing examples: `as` and `qualified`
 have special meaning only in `import` statements; `forall` and `.` only
 have special meaning in type expresssions; etc. In general, it's a good
 idea to keep keywords to a minimum; otherwise the language becomes
 increasingly awkward to use.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10756#comment:10>
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