[GHC] #11835: ApplicativeDo failed to desugar last line with pure $ <expr>

[GHC]

#11835: ApplicativeDo failed to desugar last line with pure $ <expr>
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        Reporter:  Cosmia            |                Owner:  simonmar
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:  8.0.2
       Component:  Compiler          |              Version:  8.0.1-rc2
      Resolution:                    |             Keywords:  ApplicativeDo
Operating System:  MacOS X           |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11607            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by chreekat):

 Would it possible for the docs to give a brief description of *why* one
 must follow these syntactic rules?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11835#comment:4>
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