Re: Hidden Secrets of PERL

I would try unravelling stuff in the Obfuscation section.

BTW congratulations. It has been a long time since something was pointed out to me about Perl that truly made me go, Ick! You succeeded.

BTW trading oddities, why do the following two programs print different things?

#######
## A ##
#######
use strict;
my $foo;

$foo = "outside";
for $foo (qw(a b c)) {
  print_foo();
}

sub print_foo {
  print "$foo\n";
}
__END__


#######
## B ##
#######
use strict;
our $foo;

$foo = "outside";
for $foo (qw(a b c)) {
  print_foo();
}

sub print_foo {
  print "$foo\n";
}
__END__
[download]

Comment on Re: Hidden Secrets of PERL Download Code

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Re^2: Hidden Secrets of PERL by Hue-Bond (Priest) on Oct 11, 2006 at 22:58 UTC
If the variable that is going to hold each value in an loop already exists as a lexical one (first example), foreach creates a new lexical whose scope is the loop block. If it exists as a package global (second example), foreach localizes it, thus making its scope dynamic. That's why second example's `print_foo` sees the dynamic content. -- David Serrano	[reply] [d/l]
Re^3: Hidden Secrets of PERL by bart (Canon) on Oct 12, 2006 at 11:08 UTC
Indeed and this is documented in perlsyn under the heading "foreach loops": The `foreach` loop iterates over a normal list value and sets the variable VAR to be each element of the list in turn. If the variable is preceded with the keyword `my`, then it is lexically scoped, and is therefore visible only within the loop. Otherwise, the variable is implicitly local to the loop and regains its former value upon exiting the loop. If the variable was previously declared with `my`, it uses that variable instead of the global one, but it's still localized to the loop. This implicit localisation occurs only in a `foreach` loop. OTOH, it doesn't seem to be really localized, instead, it appears to be a totally different lexical.	[reply] [d/l] [select]
Re^2: Hidden Secrets of PERL by fenLisesi (Priest) on Oct 12, 2006 at 08:49 UTC
Allow me to toil in some scrivener's work. In the "Non-Lexical Loop Iterators" section of PBP, the oracle speaks: Always declare a `for` loop iterator variable with `my`. and goes on to explain that, if there happens to be a lexical variable with the same name as the loop variable declared before the loop, that lexical variable is not reused within the loop, but a brand new independent lexical variable is implicitly created to take effect within the scope of the loop alone. Quoting the discussion there: This behaviour is contrary to all reasonable expectation. Everywhere else in Perl, when you declare a lexical variable, it is visible throughout the remainder of its scope, unless another explicit `my` declaration hides it. Update: wfsp sent me a link to this related node.	[reply] [d/l] [select]
Re^3: Hidden Secrets of PERL by Anonymous Monk on Oct 12, 2006 at 10:05 UTC
The reason is history. The ability to write: `foreach my $variable (LIST) {...}` [download] was relatively recent (5.004 I think). Before one had to write `my $variable; foreach $variable (LIST) {...}` [download] As usual, it's combining old Perl DWIM, new features and backwards compatability that leads to oddities like this. Luckely, for most people, it all just works, and old programs don't break.	[reply] [d/l] [select]