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Re: Re: Help needed understanding global variables in Perl

by rbrito (Acolyte)
on Mar 06, 2002 at 02:42 UTC ( #149595=note: print w/ replies, xml ) Need Help??


in reply to Re: Help needed understanding global variables in Perl
in thread Help needed understanding global variables in Perl

Your first program doesn't work with warnings and strict turned on and I guess that this was what I asked in the original post (I was the original poster): the interpolation of $foo in your print function is illegal, since foo is only valid within the sub foo.

Also, you say that:

Defining a global variable would require you to do: my $foo in the main scope of a 'package'.
I guess that you meant declaring a global variable, right?

It seems that using my $foo in package main doesn't seem to interfere with $main::foo anyway as can be seen in the following code snippet:

#!/usr/bin/perl use strict; use warnings; use diagnostics; package main; my $foo = "hello"; $main::foo = "world"; print "$foo $main::foo\n";
(Isn't it weird? It is terrible to think one knows the language after reading a lot about it, but still fail to understand things which should be simple semantics of the language. I would guess that the my $foo under the package main would manipulate the symbol table such that $foo were equal to $main::foo).

If $foo and $main::foo were the same variable, then the program would print world world, which it does not.

Furthermore, if you fully qualify your variables in package main, then you don't need to worry with declaring them with my, nor with our, nor with use vars, as can also be seen in the program above (see that $main::foo is not declared; yet, the compiler doesn't complain about it, even though the program is running under use strict).


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Re: Re: Re: Help needed understanding global variables in Perl
by Sinister (Friar) on Mar 06, 2002 at 09:27 UTC
    It seems you are completely correct. I was under the distinct assumption that defining a variable in a scope (aka declaring) trough the use of our, lifted that variable out of the scope and accesible for other scopes. This seems only trough when using no strict 'vars' (but that would not be nice).

    <off-topic>
      Does it matter whetter we call 'this' defining or declaring? I know the difference between the two, and if you are a nice perl -wT && strict programmer you define and declare all at once, to prevent warnings.
    </off-topic>

    Your snippet leads to the suspicion that more evil-hack's are possible with perl, then I so far realized.
    I off course tested your snippet, and it works prima!
    However if you would add these lines:
    package not_main; { print $foo; print $main::foo; print "\n"; } package main;
    It would _still_ work fine. This puzzles me. (Why one would define more packages inside one script puzzles me as well)

    As you I was somewhat confinced that I knew the language and the behavior of the interpreter by now. This has once again proofed me wrong, and that is what keeps perl interesting!

    And then, last but not least: thnx 4 ur reply ;-)

    A righthanded-lefthand....
    "Field experience is something you don't get until just after you need it."

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