[GHC] #13613: Simplify fixIO

[GHC]

#13613: Simplify fixIO
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           Reporter:  dfeuer         |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  8.4.1
          Component:  Core           |           Version:  8.0.1
  Libraries                          |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Now that we have `noDuplicate#`, I think we can make `fixIO` much simpler,
 and perhaps also faster, using something like what we do for `ST`.

 {{{#!hs
 data Res a = Res (# State# RealWorld, a #)

 fixIO f = IO $ \s ->
   let Res r@(# _s', a #) = unIO (noDuplicate >> f a) s
   in r
 }}}

 If `f` forks an `IO` thread demanding `a`, the `noDuplicate` should ensure
 that the computation is not re-entered, and the thread should wait for the
 main computation to complete.

 So I believe, anyway. The comment about eager blackholing here is a bit
 vague and doesn't give an example.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13613>
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