[GHC] #13557: error found hole: _ is not appropriate in the following case

[GHC]

#13557: error found hole: _ is not appropriate in the following case
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        Reporter:  vanto             |                Owner:  (none)
            Type:  feature request   |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:  invalid           |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by simonpj):

 * status:  new => closed
 * resolution:   => invalid


Comment:

 Here's a simplified version
 {{{
 f' = [True | s <- [_, _]]
 }}}
 Here `_` is not a wildcard pattern, as you say.  It's an underscore in an
 expression, which GHC treats as a wildcard ''expression''.  See
 [http://downloads.haskell.org/~ghc/master/users-guide/glasgow_exts.html
 #typed-holes Typed Holes] in the user manual.

 I think this is correct behaviour.  Re-open if you disagree.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13557#comment:1>
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