If you don't do timely destruction, HowMany will return the wrong value.
Well, no, not really. We'll return the right value, which is the number
of not-dead objects. That the number is surprising to the programmer
is arguably rather sub-optimal but not flat-out wrong. (Yes, I know I'm
arguing twiddly bits here)
I wouldn't call that twiddling of bits. You're redefining English.
You ask for cases where the proposed implementation breaks a program.
I give you an example of something that isn't at all contrived, and you
dismiss it as "it's not broken, it just does something surprising".
I'm glad the attitude of p5p isn't this way, any bug report could be
dismissed this way.
Why did you ask the question anyway?
The class will have the option of forcing a sweep for dead objects if
it so chooses, so there would be a fallback. Yes, this is definitely
sub-optimal, and is just a variant on the weak-ref problem. (i.e. a hack
of sorts to get around implementation issues)
I don't like any implementation issues surfacing in the language. But
it's certainly worse than the circular references problem you have with
refcounting. Then when and whether or not your DESTROY method is called
is predictable. It's properly defined when it's called, as soon as the
last reference to it disappears. With the proposed implementation, when
DESTROY is being called becomes unpredictable.
#!/usr/bin/perl
use strict;
use warnings 'all';
package Foo;
sub new {
my ($class, $ref) = @_;
bless [$ref] => $class;
}
sub DESTROY {
my $self = shift;
print $self -> [0] [-1], "\n";
}
package main;
my $ref = [];
foreach my $c (qw /one two three four/) {
push @$ref => $c;
my $obj = Foo -> new ($ref);
}
print "Exit\n";
__END__
one
two
three
four
Exit
But what will this do if
DESTROY is being called at random
times? What can, and what can't you do? Will
DESTROY be
like unsafe signal handlers?
However, it's not just the semantics you should worry about. Performance
might depend on its as well. More than once I've written code where
in the DESTROY method a file is closed (and hence, a lock on it is
released). If the DESTROY no longer is timely, the semantics won't change
- the program would still act correctly.
But the performance would no longer be acceptable, as other programs
don't get their locks fast enough.
I'm not sure that's much of a problem. The delay in destruction should
be on the order of milliseconds under most circumstances. If that's an
unacceptable delay, then odds are something more explicit than relying on
automatic destruction is in order.
Do you have anything to back up the "I think", "most circumstances" and
"odds are"? If 5 out of 100 planes have a bomb on board, then odds are
that you arrive safely, under most circumstances there's no bomb on board.
But you won't see a huge rush in plane tickets.
Perl 5 programs won't have as timely a DESTROY as they do running on the
perl 5 engine. Optionally forcing DESTROY checks at block boundaries
will be doable, and I suppose we can optionally force it at statement
boundaries, though that will be slow.
Ah "slow", the magic word that can stop anything. Refcounting is also
slow, but that didn't stop Perl from being useful for 14 years. *Anything*
is slow, for some value of slow. But the common behaviour apparently
wasn't slow enough for people to not use OOP. In fact, it was remarkably
succesful. Now, if perl6 will be so much slower than perl5 it has to let
go of useful features just to keep up, I think we can do without perl6.
Allocation failure (for example running out of filehandles) will trigger
a GC sweep and retry of the failing operation, so your program won't
run out of things for lack of timely cleanup.
Could you elaborate on that? If program1 (not necessarely written in Perl)
has an allocation failure because program2 (written in Perl) hasn't done
a GC sweep yet, how's that going to trigger a GC sweep in program2?
This only applies within a running parrot program of course--we're not
in a position to affect the behaviour of other programs. It's only when
there's a potentially recoverable resource allocation within a program
that we can do this. Locks, for example, can be tried in non-blocking
mode and, if that fails, a GC sweep can be done and the lock retried in
blocking mode to wait on other programs that might have it allocated. File
open failures due to a lack of filehandles can similarly be tried, a GC
run tried, then retried and only then throwing an exception if there are
still no filehandles.
I understand all of that. But I've been using UNIX for almost 20 years,
and I know that running just one task on a machine is an exceptional
case. Programs have to compete for resources. Nice programs let resources
go as soon as possible, and don't use more than necessary. Nice languages
allow for nice programs to be written. C is a nice language. Perl is nice,
although it gobbles up a lot of memory. Java isn't a nice language.
And it looks like Perl6 is going to be further away from C, and closer
to Java.
Java's unpredictable GC is already giving lots of people a headache when
dealing with long running Java programs. It would be a pity if Perl goes
that way too.
Could you elaborate on this? There are always tradeoffs when making
choices, and cleaner/faster internals and no worries about circular
garbage is the tradeoff for guaranteed destruction timing. If there are
more serious ramifications, it'd be good to know so we can take steps
to ameliorate them.
Well, if you find my ramifications about unpredictability not enough,
there isn't much left to discuss, is there?
Abigail, who only seems more reasons not to use perl6.
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