[GHC] #12666: ApplicativeDo fails to sequence actions

[GHC]

#12666: ApplicativeDo fails to sequence actions
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        Reporter:  bgamari           |                Owner:  simonmar
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.2
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by bgamari:

@@ -37,1 +37,1 @@
- catch this the fact that the result of the first `action` is unbound and
+ catch the fact that the result of the first `action` is not bound and

New description:

 Consider the following,
 {{{#!hs
 {-# LANGUAGE ApplicativeDo #-}
 module Fail where

 data P a = P

 instance Functor (P) where
     fmap _ P = P

 instance Applicative (P) where
     P <*> P = P
     pure _ = P


 action :: P Int
 action = P

 works :: P (Int, Int)
 works = do
     a <- action
     b <- action
     return (a,b)

 thisWorks :: P Int
 thisWorks = action *> action

 -- It seems like this should be equivalent to thisWorks.
 shouldThisWork :: P Int
 shouldThisWork = do
     action
     action
 }}}

 It seems to me that `thisWorks` and `shouldThisWork` are equivalent, yet
 the latter fails to typecheck. It seems that `ApplicativeDo` fails to
 catch the fact that the result of the first `action` is not bound and
 therefore can be sequenced with `*>`.

--

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12666#comment:2>
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