[GHC] #15143: Passing an IO value through several functions results in program hanging.

[GHC]

#15143: Passing an IO value through several functions results in program hanging.
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        Reporter:  Burtannia         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by danilo2):

 @Burtannia, thanks for the reply! Why do you think `mr` is not strictly
 evaluated there? It is a strict function argument, so it have to be
 evaluated at least to WHNF. This is State over IO, so probably there could
 be some unevaluated parts, like the inner tuple (not the tuple values,
 because this is a strict state) in the state monad definition, but in such
 case I don't see (yet) how it could affect the performance and why `!r <-
 mr` and `!_ <- mr` behave differently – they both force evaluation of all
 intermediate structures of the monadic stack.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15143#comment:9>
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