[GHC] #11760: runST with lazy blackholing breaks referential transparency

[GHC]

#11760: runST with lazy blackholing breaks referential transparency
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        Reporter:  Yuras             |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      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 Yuras):

 * failure:  None/Unknown => Incorrect result at runtime


Comment:

 Yes, I agree that it is specific to lazy `ST`. Strict one serializes
 access to mutable refs, and entering the thunk twice just creates a fresh
 set on refs.

 Re performance in presence of `noDuplicate#`: I don't have a use case for
 lazy `ST` actually, I just found the snippet on twitter. But it looks like
 lazy `ST` is useless because of the issue, so probably trading performance
 for correctness makes sense here.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11760#comment:4>
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