Hi Monks,

During development, I noticed a behavior of eval that I had not expected

use strict;
use warnings;

my %h = (a => 1, b => 2, c => 3);

sub clone_h { print "cloning \%a\n";
              return +{%h}; }

while (my ($key, $val) = each(%{clone_h()})) {
  print("$key => $val\n");
}

[download]
This results in an infinite loop, printing 
cloning %a
b => 2
cloning %a
c => 3
cloning %a
a => 1
cloning %a
c => 3
cloning %a
a => 1
cloning %a
a => 1
cloning %a
b => 2
cloning %a
c => 3
...

[download]

Obviously, clone_h() is called again and again. Of course, I can work around this by first saving the result of clone_h() in a temp variable and then put the variable into the each(). But why does each() not just evaluate its argument and then work on the result?

I tried this with perl 5.10, 5.14 and 5.38.

But: The corresponding code using an array and foreach() works as expected:

use strict;
use warnings;

my @a = qw(a b c);

sub clone_a { print "cloning \@a\n";
              return [@a]; }

foreach my $entry (@{clone_a()}) {
  print "$entry\n";
}

[download]

Why does each (but not foreach) it work like that? Why is a temporary variable forced (which seems redundant to me)?

2023-12-06 Retitled by Discipulus, as per Monastery guidelines
Original title: 'Why does eval() always re-evaluate its argument?'

In reply to Why does each() always re-evaluate its argument? by Darkwing

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