[GHC] #11607: ApplicativeDo easily foiled with `pure`

[GHC]

#11607: ApplicativeDo easily foiled with `pure`
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:  bgamari
            Type:  bug               |               Status:  patch
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  8.0.1-rc2
  checker)                           |
      Resolution:                    |             Keywords:  ApplicativeDo
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:  ado/T11607
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1931
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 Yikes!  there is no user-manual documentation about the magical behaviour
 of `return`.  Adding more magic (`pure` or `return`) sees uncool. E.g. if
 I have
 {{{
 }}}
 then using `blah` will surprisingly fail.  Simon M?

 Regardless whatever the behaviour is ought to be carefully specified in
 the user manual.

 So I'm not complaining about Phab:D1931, but rather asking for proper
 documentation.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11607#comment:5>
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