What would Squeak be like without non-local returns
andreas.raab at gmx.de
Wed Nov 7 06:23:33 UTC 2007
Igor Stasenko wrote:
> Let suppose that Mr.Satan will rewrite an example above as following:
> object eventual ifTrue: [100 atRandom seconds asDelay wait. ^3].
> 100 atRandom seconds asDelay wait.
> When, in real case, a random delay could be replaced by any code which
> needs a time to evaluate.
> What is the answer then? :)
Same answer (though strictly speaking you may get an error that you
can't *wait* in an eventual system). The reason being that <object> will
only get an eventual ref to the block so it will only be able to send
other eventual messages to it. Put differently, ifTrue: would need to be
"I am true, so evaluate aBlock"
aBlock isBlock "if it's an immediate block..."
ifTrue:[aBlock value] "... evaluate it directly ..."
ifFalse:[aBlock eventual value] "... otherwise eventual"
(I'm ignoring the issues of meta-circularity for ifTrue: here; the above
is purely conceptual) The main point is that because the evaluation of
the block would be eventual it would *necessarily* evaluate after the
immediate delay and the entire message has completed *regardless* of how
long or how short that delay is. If that were different it would violate
one of the fundamental axioms of event-loop concurrency (the fact that
only one message is ever executed at the time) and wouldn't count as
message passing concurrency anymore.
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