[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):  Phab:D2345
       Wiki Page:                    |
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Comment (by chreekat):

 Comment 6 provides the rationale for this situation, where the expected
 behavior isn't really possible because of lower-level concerns.

 Could that rationale be mentioned in the docs? Something like:


 {{{
 --- a/docs/users_guide/glasgow_exts.rst
 +++ b/docs/users_guide/glasgow_exts.rst
 @@ -860,7 +860,9 @@ upon the results ``p1...pn`` with either ``return`` or
 ``pure``.

  Note: the final statement really must be of the form ``return E`` or
  ``pure E``, otherwise you get a ``Monad`` constraint.  Using ``$`` as
 -in ``return $ E`` or ``pure $ E`` is also acceptable.
 +in ``return $ E`` or ``pure $ E`` is also acceptable. This is because we
 +must spot the ``return`` before `GHC transforms do-syntax into fmap
 +<https://ghc.haskell.org/trac/ghc/ticket/11835?replyto=6#comment:6>`_.
 }}}

 I've not had much experience contributing to GHC, so I apologize for the
 unwieldy format of this suggestion.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11835#comment:10>
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