[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:7 goldfire]:
 > But the only way to bind a variable is in a pattern. That's why it must
 become a full keyword, in my view. Or have I missed something?

 Sorry, to be more explicit - `impossible` only has special meaning in
 pattern expressions, and only '''after''' the ''var'' term, which always
 occurs in first position in pattern expressions that contain it.

 We can simplify that to say that `impossible` never has special meaning in
 first position in a pattern expression in any case, since anyway that
 never makes sense.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10756#comment:8>
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