[GHC] #9388: Narrow the scope of the notorious "state hack"

[GHC]

#9388: Narrow the scope of the notorious "state hack"
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        Reporter:  simonpj           |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by nomeata):

 Ah, I think it might have been me, at least partially:

 In `MkId`, the `realWorldPrimId` has `setOneShotInfo` stateHackOneShot
 set. This means that _every_ lambda with an argument of type `State#` is
 marked as oneshot. This was introduced in changeset:80989de/ghc – maybe
 this bit was not even meant to be merged?

 Back then, the `OneShotInfo` would not be exported, so the unfolding for
 `hPutStr2` would say `(\ s ::String eta :: State# RealWorld -> ...` But
 when I added the `oneShot` magic function, I also made sure that the
 `OneShotInfo` would be written to the interface, so it now says `(\ s
 ::String eta :: State# RealWorld[oneShot] -> ...` and the simplifier will
 float in the call to `reverse`.

 I’ll rip out all state hack code (which seems to be implemented by
 `OneShotInfo` on `realWorldPrimId` and via `typeOneShot`) and see if I can
 achieve something similar in a cleaner way and only for top-level bindings
 in the cardinality analysis. It’s a nice weekend relaxation from paper
 proofreading :-)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9388#comment:5>
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