[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:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by duncan):

 Can I suggest a closely related idea, and also related to #9520

 {{{
 data Pipe i o r = Yield o {-# NOUPDATE #-} (Pipe i o r)
 }}}

 This says we'll never do thunk updates on that field in that constructor.
 So similar idea (I believe) to `oneShot` lambdas.

 Indeed we might need both no update on fields and oneShot, I'm not sure,
 e.g.:

 {{{
 data Pipe i o r = Yield o {-# NOUPDATE #-} (Pipe i o r)
                 | Await   {-# NOUPDATE #-} (Either r i -> Pipe i o r)

 -- smart constructor:
 await f = Await (GHC.Magic.oneShot f)
 }}}

 What's all this for? For avoiding treating these control structures as
 data structures (which is what #9520 is all about).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12620#comment:5>
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