From: Campbell, John (John.Campbell_at_email.domain.hidden)
Date: 2001-09-10 21:46:32
Peter wrote:
> BUT ... it really screws up some basic algebraic laws! For example:
>
> P = PAR = PAR
> P SKIP
> SKIP P
>
> where the = means semantic equivalence. But, on the LHS, P's
> exception is
> thrown. On the middle/RHS, it isn't! So, with exceptions,
> that law no longer
> holds :-( ...
It seems to me that the discrepancy is resolved if PAR were to handle
exceptions of subprocesses the same way the OS does. Ie if poisoning
P causes the OS to print a message for the LHS, it should also cause
the same behavior in PAR (perhaps delayed until sub processes terminate).
> The good thing about this is that it gives us a safe way of
> freezing processes
> temporarilly or permanently. The bad thing is that, in the
> latter case, it
> doesn't permit any tidy-up actions (that involve further
> synchronisation).
I think the way out of this dilemma is to view all processes as
having a default handler. If you need tidy-up operations,
replace the default handler with explicite code. (Or, if you're
a stickler for WYSIWYG, require all processes to explicitly
handle poison. Personally, I like the former)
What do you think?
-jc
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