[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):

 Fair enough, that's an easier workaround. But the idea is to have
 something a little more compositional. For example, in the case of
 conduits, we probably ''never'' want to share a conduit value. So it would
 be great if we could annotate the conduit constructors with a noupdate
 annotation, and then users of the conduit library don't have to worry
 about this problem anymore. After all, in the list example, it's not
 obvious that

 {{{#!hs
 main :: IO ()
 main = retry $ runConduit someConduit
 }}}

 has a space leak; even less so when that retry and the runConduit are in
 different places:

 {{{#!hs
 go :: IO ()
 go = runConduit someConduit

 main :: IO ()
 main = retry go
 }}}

 We'd need to have the foresight to write

 {{{#!hs
 main :: IO ()
 main = retry $ return () >>= \_ -> go
 }}}

 The situation really is very close to strictness; do we want to make sure
 every single function using a datatype has the right seqs in the right
 place, or we just put some strictness annotations on the datatype?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12620#comment:13>
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