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

18 Feb 2016

      #11607: ApplicativeDo easily foiled with `pure`
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  8.0.1-rc2
  (Type checker)                     |
           Keywords:  ApplicativeDo  |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 `ApplicativeDo` fails to desugar `do` blocks ending with `pure`. For
 instance,
 {{{#!hs
 {-# LANGUAGE ApplicativeDo #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}

 newtype MaybeA a = MaybeA (Maybe a)
                  deriving (Show, Functor, Applicative)

 main = print $ do x <- MaybeA $ Just 42
                   pure x
 }}}

 However, if the final `pure` is changed to `return` things work as
 expected.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11607
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