[GHC] #12610: Emit tab warning promptly

[GHC]

#12610: Emit tab warning promptly
-------------------------------------+-------------------------------------
        Reporter:  dfeuer            |                Owner:  dalaing
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.3
       Component:  Compiler          |              Version:  8.0.1
  (Parser)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dalaing):

 It looks like getting this to work while keeping the ability to run the
 parser with a pure function might involve modifying the `PFailed`
 constructor from the `ParseResult` data type.

 It's not the end of the world, but I figure it's worth mentioning in case
 folks have objections.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12610#comment:7>
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