[GHC] #12620: Allow the user to prevent floating and CSE

[GHC]

#12620: Allow the user to prevent floating and CSE
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        Reporter:  nomeata           |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9520, #8457      |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by edsko):

 Replying to [comment:25 simonpj]:
 > > Any thunk of type `Sink` never gets updated.
 > Notice the "`-o`", meaning a "one-shot function".  The idea is that one-
 shot functions are called at most once.  (Maybe exactly once, but I think
 at-most once is better.)
 >
 > So in your `countFrom` example, the continuation `k` would not be
 floated outside the lambda; and if it was written outside it'd be floated
 inside the lambda.
 >
 > GHC already has the notion of a one-shot lambda; it's just not dignified
 as part of the type system.
 >
 > Would that serve?  I think that you do intend that the argujment of
 `Await` is called at most once, don't you?

 Typically, yes, but not necessarily. After all, in the minimal example
 above, ''if'' the exception handler gets executed then the whole process
 starts over. Ideally it would start over with a newly constructed conduit,
 but if we cannot prevent sharing, it would start over with the same
 conduit.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12620#comment:26>
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