[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 simonpj):

 It is nothing to do with strictness!  Consider
 {{{
 foo :: IO Int -> IO Int
 foo a = do { i1 <- a
            ; i2 <- a
            ; return (i1+i2) }
 }}}
 This function must perform `a` twice.  It is ''not'' ok to "optimise" it
 to
 {{{
 foo a = do { i1 <- a
            ; return (i1+i1) }
 }}}
 Reason: `a` might have side effects (which must be performed twice) and it
 might return different values on its successive calls.

 In the example of this ticket, in `incTime` the action `o` must be called
 twice.  And in the particular example, the action is another call to
 `incTime` which calls its `o` twice; and so on.  Result: 2^n calls of the
 final action, hence exponential behaviour.

 Strictness and bangs have no effect on this story

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15143#comment:10>
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