Re^5: Lexical closures

[backstab]

I think we can also show how the aliased variable can be global with the following,

use vars '$i';
$i = 123;

sub foo {
  my $arg = shift;
  print "$arg, $i\n";
}

foreach $i (0..2) {
  foo($i);
}
foo('x'); # obviously alias works only within the loop

# Prints,
# 0,0
# 1,1
# 2,2
# x,123

If so, I have a question related to my scope. Within the previous example let's declared $i with my instead.

my $i;
$i = 123;

sub foo {
  my $arg = shift;
  print "$arg, $i\n";
}

foreach $i (0..2) {
  foo($i);
}
foo('x');

# Prints,
# 0,123
# 1,123
# 2,123
# x,123

At this point I'm confused in fact. Should not we get the same result? Does not my scope to nested blocks as well?

It looks like aliasing does not affect my'd variable despite to be within the same scope or nested scope. The previous result is what I have expected using my on the foreach itself.

[ikegami]

In the second snippet, foo() captures $i since $i is a lexical. That means the $i in foo() will always refer to the specific SV associated with $i when foo was compiled. When the loop aliases $i to a different SV, it doesn't affect foo.

use Devel::Peek;

my $i;          # Lexical variable

sub foo { Dump($i); }

Dump($i);       # SV ... at 0x226048  \
for $i ( 1 ) {  #                      > same
   foo();       # SV ... at 0x226048  /
   Dump($i);    # SV ... at 0x226d8c
}

In the first snippet, there is no closure since $i is not a lexical. That means the foo() will always grab the current value of $main::i.

use Devel::Peek;

our $i;         # Package variable

sub foo { Dump($i); }

Dump($i);       # SV ... at 0x226d94
for $i ( 1 ) {
   foo();       # SV ... at 0x226d7c  \ same
   Dump($i);    # SV ... at 0x226d7c  /
}