[GHC] #13875: ApplicativeDo desugaring is lazier than standard desugaring

[GHC]

#13875: ApplicativeDo desugaring is lazier than standard desugaring
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:  ApplicativeDo
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonmar):

 Wow, you're right.  I hadn't realised this, and I think you're the first
 person to point it out.

 It's not limited to `Functor`, the problem occurs with `Applicative` too:

 {{{
 Prelude Data.Maybe> :set -XApplicativeDo
 Prelude Data.Maybe> let f m = do () <- m; () <- m; return ()
 Prelude Data.Maybe> :t f
 f :: Applicative f => f () -> f ()
 Prelude Data.Maybe> isJust (f (Just undefined))
 True
 }}}

 To fix this properly we would have to prevent `ApplicativeDo` from
 applying to any statement with a strict pattern match.  In practice I
 doubt anyone is going to write `~p` to make `ApplicativeDo` work, but
 fortunately it still works for simple variable patterns.

 Ugh, I don't know what the best fix is here, but I agree at the very least
 we need some documentation.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13875#comment:3>
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