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Message from discussion Autovivi
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Deven T. Corzine  
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 More options Aug 13 2002, 5:48 pm
Newsgroups: perl.perl6.language
From: de...@ties.org (Deven T. Corzine)
Date: Tue, 13 Aug 2002 16:47:55 -0400 (EDT)
Local: Tues, Aug 13 2002 4:47 pm
Subject: Re: Autovivi
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On Tue, 13 Aug 2002, Nicholas Clark wrote:
> On Tue, Aug 13, 2002 at 03:06:40PM -0400, Deven T. Corzine wrote:

> > The only accurate way to know if the code modifies the variables is to do
> > some sort of dataflow analysis, and it can't be 100% accurate even then.  
> > (Suppose a "shift" may or may not happen, depending on the parameters, then
> > $_[2] is modified?)  Of course, it should often be possible (at least in
> > principle) to determine that it's impossible for a particular parameter to
> > be modified, and play it safe by assuming "is rw" for undeterminable cases.

> Well, perl5 does already manage to avoid auto-vivifying hash keys when they
> are used as subroutine arguments. It uses magic, rather than dataflow
> analysis:

Yes, "magic" obviously can work, but it's a greater run-time penalty -- if
you do some sort of dataflow analysis, you pay the penalty at compile time
instead.  I'm not sure which is better, ultimately.  It probably varies.

Anyway, the Perl 5 magic you speak of presumably only works for a single
level of hash referencing -- we were specifically discussing a multilevel
reference, which immediately creates missing substructures in Perl 5, even
used in a read-only rvalue context in a simple Perl 5 expression...

Deven


 
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