[GHC] #15349: fixST is a bit wrong

[GHC]

#15349: fixST is a bit wrong
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        Reporter:  dfeuer            |                Owner:  dfeuer
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Core Libraries    |              Version:  8.5
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by dfeuer):

 * owner:  (none) => dfeuer


Comment:

 I raised this issue on the libraries list. Levent Erkök was
 [https://mail.haskell.org/pipermail/libraries/2018-July/028888.html first
 to respond]. He recognized that this bug leads to a violation of the
 `MonadFix` laws, and surprising behavior, but that it was harmless enough
 to get away with just documenting it. Then Arseniy Alekseyev came up with
 [https://mail.haskell.org/pipermail/libraries/2018-July/028889.html a more
 frightening example], in which this feature of `fixST` turns safe code
 using unsafe operations into unsafe code. That was enough to
 [https://mail.haskell.org/pipermail/libraries/2018-July/028892.html tilt
 Edward Kmett] toward fixing.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15349#comment:2>
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