Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

How is it that I am able to see the unqualified $var variable from within the Bar package, as shown below (last line)?

#!/usr/bin/env perl

use Modern::Perl;

package Foo;

our $var = 42;  # This is $Foo::var.

# Change the current package.
package Bar;

# This works, as expected.
say "\$Foo::var: $Foo::var";

# But why does this work? $var is not in package Bar,
# and we are in package Bar here.
say "\$var: $var";  # !?

[download]
  • Comment on How is it that I can see this package variable from another package without fully-qualifying its name?
  • Download Code

Replies are listed 'Best First'.
Re: How is it that I can see this package variable from another package without fully-qualifying its name?
by chromatic (Archbishop) on Apr 24, 2011 at 07:30 UTC

    An our (see especially the first paragraph) binding respects scope—in this case, file scope.

[reply]

      I don't know exactly what you mean when you say that an our binding "respects scope".

      I do see that the declaration and definition of $var is at file scope. My understanding is that "package Bar" changes the package from Foo to Bar. Maybe I'm confusing "change of package" with "change of scope"?

      I see that the following works as I'd expect:

      #!/usr/bin/env perl

use Modern::Perl;

{
    package Foo;

    our $var = 42;  # This is $Foo::var.
}

{
    package Bar;

    # This works, as expected.
    say "\$Foo::var: $Foo::var";

    #say "\$var: $var";  # Fails, as expected.
}

#say "\$var: $var";  # Fails, as expected.

say "\$Foo::var: $Foo::var";

      [download]

      Does that look like the correct way to keep packages separate in a single file?

      (I realize that everything would also work as expected if I kept each package in its own module. And it would make them easily testable as well.)

[reply]
[d/l]
        Maybe I'm confusing "change of package" with "change of scope"?

        Exactly! The package keyword doesn't introduce a new scope for my or our bindings. Only a new file or curly braces do.

        Keeping each package in a separate file is generally the best approach, but using curly braces to isolate packages works well too. Perl 5.14 has a new package NAME { ... } syntax to make this even easier.

[reply]
[d/l]
Re: How is it that I can see this package variable from another package without fully-qualifying its name?
by tchrist (Pilgrim) on Apr 24, 2011 at 20:07 UTC
    You aren’t, because it isn’t a package variable. As the documentation says, an our declaration creates a lexically scoped alias to a package variable. That alias is of course tied therefore to a scope, not to a package.

    Since it is lexically scoped, there is no reason that changing the default prefix used for global variables — read, the package — should have anything at all to do with changing scope. Scoped variables are completely different, since they have no package prefix.

[reply]
Re: How is it that I can see this package variable from another package without fully-qualifying its name?
by LanX (Saint) on Apr 24, 2011 at 20:50 UTC
[reply]
Re: How is it that I can see this package variable from another package without fully-qualifying its name?
by John M. Dlugosz (Monsignor) on Apr 24, 2011 at 11:17 UTC
    Please: it's its, not it's in the title!

    Here's a good way to remember it: qw/his hers its/ none of them have apostrophes in them. Whenever you see "it's" mentally expand it into "it is", since that's what it always is.

[reply]

      Arg! Can't believe I did that. I get it right almost all of the time, but sometimes it still manages to trick me: "Joe's bar and grill, Larry's language, it's scope ... {sigh}"

[reply]
        If you logged in rather than posting as Anonymous, you could edit it and fix it after you see it. That bugs me more than the mistake itself; the fact that it's cast in stone forever now.

        So, how about signing up and getting your own user name?

        "Joe's bar and grill, Larry's language, it's scope ... {sigh}
        Because you are mixing proper nouns and pronouns. A better way to remember is Hi's bar a grill, he'r language, it's scope... er, huh? His/her/its form a set.
[reply]

Back to Seekers of Perl Wisdom