[GHC] #9723: Give Tab warning only once per file

[GHC]

#9723: Give Tab warning only once per file
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        Reporter:  nomeata           |                   Owner:  dalaing
            Type:  feature request   |                  Status:  patch
        Priority:  low               |               Milestone:
       Component:  Compiler          |                 Version:  7.9
      Resolution:                    |                Keywords:  newcomer
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:  Phab:D760
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Comment (by goldfire):

 While I generally agree with @hvr in wanting to think about tooling, I
 disagree in this instance. It's ''very'' easy for tooling to find tabs on
 its own -- much easier, I would bargain, than trying to parse GHC's
 warnings. And maintaining two different warnings, with their own flags,
 for "warn once" vs. "warn every time" seems heavy.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9723#comment:5>
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