Re: wantarray and Tied Hashed
On Thu, May 24, 2001 at 04:35:52PM +0100, Cross David - dcross wrote:
my @array = $h{two};
I bumped into this in 1997 and became convinced list contexts aren't
propagated to the effective sub call. If you look at the above line,
there's something very odd seeming about it anyway, and it's not
necessarily clear WTF is going on :-) At that time I didn't really have
resources/time to mention it to anyone and worked around it (them days of
riding back to Reading at 3am or sleeping on a couch ready for a 7am
start, etc -- and I were grateful!)
The source for what I was working on is locked on an NT box (boo!) at
work; I'll check there and see what I did (FWIW; probably not exciting).
Paul
Re: wantarray and Tied Hashed
On Thu, May 24, 2001 at 11:28:27AM +0100, Cross David - dcross wrote:
my @array = $h{two};
^
In perl 5 at least, *this* is your scalar context.
--
Little else matters than to write good code.
-- Karl Lehenbauer
RE: wantarray and Tied Hashed
From: Simon Cozens [mailto:[EMAIL PROTECTED]]
Sent: Friday, May 25, 2001 12:02 PM
On Thu, May 24, 2001 at 11:28:27AM +0100, Cross David - dcross wrote:
my @array = $h{two};
^
In perl 5 at least, *this* is your scalar context.
Good point. But even if I change it to:
my @array = @h{two};
FETCH doesn't get called in list context (instead it gets called once in
scalar context for each key in the list - in this case once). I can see why
Perl does this and I _almost_ agree that it's the right thing.
Anyway, as I said before, you can work around it with
my @array = tied(%h)-FETCH('two');
If anyone is interested, Tie::Hash::Regex is currently winging its way to
your favourite CPAN mirror.
Dave...
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Re: wantarray and Tied Hashed
On Fri, May 25, 2001 at 12:15:44PM +0100, Cross David - dcross wrote:
Anyway, as I said before, you can work around it with
my @array = tied(%h)-FETCH('two');
If anyone is interested, Tie::Hash::Regex is currently winging its way to
your favourite CPAN mirror.
I wonder, could you do some magic with the calling stack so that your
FETCH can Do The Right Thing?
--
David Cantrell | [EMAIL PROTECTED] | http://www.cantrell.org.uk/david/
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Re: wantarray and Tied Hashed
On Fri, May 25, 2001 at 12:36:59PM +0100, Cross David - dcross wrote: From: David Cantrell [EMAIL PROTECTED] I wonder, could you do some magic with the calling stack so that your FETCH can Do The Right Thing? Or, I could just accept that I'm a BAD MAN who is trying to PERVERT PERL in NASTY WAYS. No, you're confusing yourself with Damian :-) -- David Cantrell | [EMAIL PROTECTED] | http://www.cantrell.org.uk/david/ Rip, Mix, Burn, unless you're using our most advanced operating system in the world which we decided to release incomplete just for a laugh
Re: wantarray and Tied Hashed
David Cantrell [EMAIL PROTECTED] writes: On Fri, May 25, 2001 at 12:36:59PM +0100, Cross David - dcross wrote: From: David Cantrell [EMAIL PROTECTED] I wonder, could you do some magic with the calling stack so that your FETCH can Do The Right Thing? Or, I could just accept that I'm a BAD MAN who is trying to PERVERT PERL in NASTY WAYS. No, you're confusing yourself with Damian :-) [FX: points to Symbol::Approx::Sub] Are you *quite* sure about that? -- Piers Cawley www.iterative-software.com
wantarray and Tied Hashed
Apologies for dragging us off-topic again...
Am I missing something obvious here?
#!/usr/bin/perl -w
use strict;
package Tie::Hash::Test;
use Tie::Hash;
use vars qw(@ISA);
@ISA = 'Tie::StdHash';
sub FETCH {
print wantarray is , wantarray ? true\n : false\n;
return $_[0]-{$_[1]};
}
package main;
my %h;
tie %h, 'Tie::Hash::Test';
%h = (one = 1, two = 2);
my $scalar = $h{one};
my @array = $h{two};
This prints out wantarray is false on both accesses. To me, this implies
that Perl is doing something strange behind the scenes and is forcing the
FETCH call to always be in scalar context, when I'd expect the second call
to be evaluated in list context.
Any clues?
Dave...
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RE: wantarray and Tied Hashed
[snip]
An update -
Calling FETCH like this:
$scalar = tied(%h)-FETCH('one');
@array = tied(%h)-FETCH('two');
Does the 'right' thing. So it's certainly something in the tie interface.
Dave...
[contemplating searching the perl source code]
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Re: wantarray and Tied Hashed
On Thu, May 24, 2001 at 11:28:27AM +0100, Cross David - dcross wrote:
package Tie::Hash::Test;
sub FETCH {
print wantarray is , wantarray ? true\n : false\n;
return $_[0]-{$_[1]};
}
package main;
my %h;
tie %h, 'Tie::Hash::Test';
%h = (one = 1, two = 2);
my $scalar = $h{one};
my @array = $h{two};
This prints out wantarray is false on both accesses. To me, this implies
that Perl is doing something strange behind the scenes and is forcing the
FETCH call to always be in scalar context, when I'd expect the second call
to be evaluated in list context.
FETCH will indeed be called in scalar context. You can only store scalars
as hash values, and so you will never want to get an array out of them.
Even if you do:
my @array=@h{@multiple_keys}
FETCH will still be called once for each key, in scalar context.
--
David Cantrell | [EMAIL PROTECTED] | http://www.cantrell.org.uk/david/
Rip, Mix, Burn, unless you're using our most advanced operating system
in the world which we decided to release incomplete just for a laugh
Re: wantarray and Tied Hashed
On Thu, May 24, 2001 at 12:28:39PM +0100, Cross David - dcross wrote:
Calling FETCH like this:
$scalar = tied(%h)-FETCH('one');
@array = tied(%h)-FETCH('two');
Does the 'right' thing. So it's certainly something in the tie interface.
Well yes, because you're just calling a plain ol' object method and so the
tie doesn't get a chance to work its magic.
--
David Cantrell | [EMAIL PROTECTED] | http://www.cantrell.org.uk/david/
Rip, Mix, Burn, unless you're using our most advanced operating system
in the world which we decided to release incomplete just for a laugh
RE: wantarray and Tied Hashed
From: David Cantrell [EMAIL PROTECTED]
Sent: Thursday, May 24, 2001 1:57 PM
On Thu, May 24, 2001 at 11:28:27AM +0100, Cross David - dcross wrote:
package Tie::Hash::Test;
sub FETCH {
print wantarray is , wantarray ? true\n : false\n;
return $_[0]-{$_[1]};
}
package main;
my %h;
tie %h, 'Tie::Hash::Test';
%h = (one = 1, two = 2);
my $scalar = $h{one};
my @array = $h{two};
This prints out wantarray is false on both accesses. To me, this
implies
that Perl is doing something strange behind the scenes and is forcing
the
FETCH call to always be in scalar context, when I'd expect the second
call
to be evaluated in list context.
FETCH will indeed be called in scalar context. You can only store scalars
as hash values, and so you will never want to get an array out of them.
Even if you do:
my @array=@h{@multiple_keys}
FETCH will still be called once for each key, in scalar context.
All true, and very valid. But you forget that we're talking about _tied_
hashes here. And this means that all bets about behaviour are off.
In this instance, I found the 'problem' when working on Tie::Hash::Regex. In
this module you can lookup a hash value using a regex. It's therefore very
likely that one regex would match a number of hash keys and in that case I'd
like to know if FETCH is called in list context so I can return _all_ of the
matching keys.
This is the first time that my imagination has come up with something that
you can't do in Perl and it's vaguely unsettling. I'm betting it's just an
oversight and if I mention it on p5p we'll get a fix pretty pronto :)
Dave...
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