This is a bit embarrassing. I thought I understood dereferencing, but in glancing at a coworke'rs code, I stumbled across an edge case I don't understand. He was using a ternary operator, but instead using it to choose what he's assigning from, he used it to choose what he's assigning to:

# assigning from:
my $foo = $bar ? $baz : $quux;

# assigning 'to
push @{ $row->{close_time} ? \@closed : \@open }, $row;

[download]

That surprised me, but it seems reasonable. But then he also pointed out that taking a reference to the arrays is not necessary:

push @{ $row->{close_time} ? @closed : @open }, $row;

I thought that @{} would dereference the contained expression. The following script further illustrates my confusion:

#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my ( @open, @closed );

my $open = 1;
push @{ $open ? @open  : @closed }  => 'boo!';
push @{ $open ? \@open : \@closed } => 'boo!';
print Dumper( \@open );

push @{ @{ @{ @{ @{@open} } } } } => ['bar'];
print Dumper(\@open);

[download]

That prints:

$VAR1 = [
          'boo!',
          'boo!'
        ];
$VAR1 = [
          'boo!',
          'boo!',
          [
            'bar'
          ]
        ];

[download]

I've been using @{} and similar constructs for so long that I can't even remember where they are documented. perlref says:

Anywhere you'd put an identifier (or chain of identifiers) as part of a variable or subroutine name, you can replace the identifier with a BLOCK returning a reference of the correct type. In other words, the previous examples could be written like this:

    $bar = ${$scalarref};
    push(@{$arrayref}, $filename);
    ${$arrayref}[0] = "January";
    ${$hashref}{"KEY"} = "VALUE";
    &{$coderef}(1,2,3);

[download]

That suggests that the block is supposed to return a reference, but clearly it's not required Is this documented behavior?

Cheers,
Ovid

New address of my CGI Course.

In reply to Misunderstanding dereferencing by Ovid

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.