[GHC] #11781: Improve common sub-expression

[GHC]

#11781: Improve common sub-expression
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2074
       Wiki Page:                    |
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Comment (by nomeata):

 Note that the definition of `Monad IO` indeed has `(>>) = (*>)` and `m *>
 k = m >>= \ _ -> k`. So previously, CSE was able to to common up  `m *> k
 = m >>= \ _ -> k` with `thenIO`, and now it does not do that any more.
 Maybe some casts got into the way?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11781#comment:8>
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