[GHC] #14249: ApplicativeDo: Pattern matching on a bind forces a Monad constraint

[GHC]

#14249: ApplicativeDo: Pattern matching on a bind forces a Monad constraint
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           Reporter:  mutantmell      |             Owner:  (none)
               Type:  bug             |            Status:  new
           Priority:  normal          |         Milestone:
          Component:  Compiler        |           Version:  8.2.1
           Keywords:  ApplicativeDo   |  Operating System:  Linux
       Architecture:  x86_64 (amd64)  |   Type of failure:  Other
          Test Case:                  |        Blocked By:
           Blocking:                  |   Related Tickets:
Differential Rev(s):                  |         Wiki Page:
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 Pattern matching on a bind in 8.2.1 confuses ApplicativeDo, and causes the
 type to have a Monad constraint where a Functor constraint would suffice.

 In GHC 8.0:

 {{{#!hs
 > :set -XApplicativeDo
 > :t \x -> do { (a, _) <- x; pure (a+1) }
 \x -> do { (a, _) <- x; pure (a+1) }
   :: (Num b, Functor f) => f (b, t) -> f b
 }}}

 In GHC 8.2:

 {{{#!hs
 > :set -XApplicativeDo
 > :t \x -> do { (a, _) <- x; pure (a+1) }
 \x -> do { (a, _) <- x; pure (a+1) }
   :: (Num b1, Monad m) => m (b1, b2) -> m b1
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14249>
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