[GHC] #12143: ApplicativeDo Fails to Desugar 'return True'

[GHC]

#12143: ApplicativeDo Fails to Desugar 'return True'
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           Reporter:  MichaelBurge   |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 If I bring in a spurious parameter, ApplicativeDo correctly infers
 Functor.
 {{{
 Prelude> :t \m -> do { x <- m; return True; }
 \m -> do { x <- m; return True; } :: Functor f => f t -> f Bool

 Prelude> :t \m -> do { m; return True; }
 \m -> do { m; return True; } :: Monad m => m a -> m Bool
 }}}

 However, without the spurious parameter, ApplicativeDo assumes monad.
 {{{
 Prelude> :t do { return True; }
 do { return True; } :: Monad m => m Bool
 }}}

 The first block in particular shows two examples that seem inconsistent
 when placed next to each other. It seems like if 'x' is unused, then 'x <-
 m' and 'm' should be equivalent.

 I would expect all of these to desugar to Functor or Applicative. And
 maybe 'return True' should be Monad, but 'do { return True; }' should be
 Applicative?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12143>
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