[GHC] #9127: Don't warn about pattern-bindings of the form `let !_ = rhs`

19 May 2014

      #9127: Don't warn about pattern-bindings of the form `let !_ = rhs`
------------------------------------+-------------------------------------
       Reporter:  refold            |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 With GHC 7.8.2, code like

 {{{
         let !_           = rqPostParams rq
         let !_           = rqParams rq
 }}}

 triggers the following warnings:

 {{{
 src/Snap/Internal/Test/RequestBuilder.hs:150:13: Warning:
     This pattern-binding binds no variables: !_ = rqPostParams rq

 src/Snap/Internal/Test/RequestBuilder.hs:151:13: Warning:
     This pattern-binding binds no variables: !_ = rqParams rq
 }}}

 I think that `let !_ = rhs` shouldn't trigger a warning, just like `let _
 = rhs` doesn't.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9127
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