Local and Global variables in perl

Question

I am having few doubts about the local/our scope in Perl. I read a lot of documentation, but I am still in confusion is there. Following are the confusions

1. What is local scope?

   what I read is -> local copies the value of global variable, change the value, user will use it and outside the block it will retain the global value

   Confusion -> my does the same thing. Only benefit I see is that some variables like $package::var cannot be declared with my scope but can be declared with local scope. What else for local

2. What is "global" variable?

What I read is -> Its scope is within the package. Basically we put the global variable in @EXPORT array and use it or append the namespace with it to use in other packages.

I doubt -> Again if we declare variable with my scope in main only then we can access the variable throughout the package. Is that right? Is it possible to add the my scoped variables in @EXPORT array and use it in another packages?

I think global variables are declared with our keyword. Is there any other way to do so?

This question may look repetitive, but I am confused.

Answer 1

In terms of scoping, there are three kinds of variables in Perl.

• Lexical variables are lexically scoped, which means they are only visible in the current lexical scope (basically file or block).

• Package variables, on the other hand, can be used using their qualified form (e.g. $Foo::x) from anywhere in the interpreter, and they can be used without qualification by any code that shares the variable's package.

• Certain package variables are visible without qualification anywhere in the interpreter. These include punctuation vars and a few named vars such as @ARGV and STDOUT. For example, $x refers to $Foo::x when in package Foo and $Bar::x when in package Bar (assuming no lexical var named $x is in scope), but $_ always refers to $::_.

Variables are destroyed when they are no longer referenced.

• Lexical variables are usually destroyed when the lexical scope is exited.

• Package variables are usually destroyed when the program exits.

Here are ways to create variable.

• my and state create a lexical variable.

• our creates a lexical variable that is aliased to the variable of the same name in the current package. In other words, our $x; is equivalent to my \$x = \$Foo::x; when in package Foo.

• Package variables are created on use.

local doesn't create any variables. It simply backs up a variable until the current lexical scope is destroyed. It is restored from its backed-up value at that point.

---

my does the same thing.

No. local does not change the scope of a variable. While a lexical variable is only visible in a lexical scope, a localized package variable is still visible across the entire interpreter.

$x = 123;
sub foo { print "$x\n"; }
{ local $x = 456; foo(); }  # 456
foo();                      # 123

$x = 123;
sub foo { print "$x\n"; }
{ my $x = 456; foo(); }   # 123
foo();                    # 123

---

What else for local

local is primarily used to approximate the functionality of my for variables that cannot otherwise be declared lexically.

Historically, that was all variables. Since 5.6, only punctuation variables cannot be declared lexically.

---

What is "global" variable?

A global variable is a variable that can seen globally.

All package variables can be seen by any code in the interpreter, so they're all global.

Or are they? To see them from other packages, you need to qualify them. Are $x and $Foo::x the same variable?

To some, global variables refers to the set of package variables you can use unqualified. It means that package changes the set of global variables. And since the package directive is usually used on a file-basis, that means file-level lexicals are also effectively global by this definition. And they are indeed called that sometimes.

But if the package changes the set of variables that are global, then they're not really global, are they? So think some people, which only consider punctuation variables (e.g. $_) and the few named variables that can be used unqualified from anywhere (*::STDOUT) to be global.

In short, it's a pretty useless term.

---

Is it possible to add the my scoped variables in @EXPORT array and use it in another packages?

No. @EXPORT is used by Exporter. Exporter would not be able to find anything but package symbols (since files are compiled in fresh lexical scopes), so @EXPORT must only contain package symbols.

Answer 2

There are two kinds of variables, lexically scoped and globally scoped.

In Perl before version 5, there was only globally scoped. These variables are the package variables. These variables are available everywhere in the program if you use the package prefix.

The local keyword was introduced to provide a way to alter the value of one of these package global variables inside a limited scope, such as inside one subroutine. It will save the old value on a stack when entering the scope with the local statement, and upon exiting, it will restore the old value. These are still package globals, which means that they are still available everywhere. If you are inside a scope with a local variable, and you call a subroutine, that variable is still visible inside that subroutine.

The my keyword was introduced in version 5, and provides lexically scoped variables. These variables only exist inside the scope where they are declared. This means that if you call a subroutine, that my variable is not visible. Upon exiting a scope, the my variables simply go away. You should prefer to use my variables when possible, because you do not want your variables to be visible inside subroutines that you call. You cannot use these type of variables in the @EXPORT list because these variables are not visible outside of their scope.

Finally, the our keyword is a combination of both, in that it gives you a variable that is a package global, but that variable is lexically scoped. This means it will be available anywhere in the program, but at the end of the enclosing block, you cannot refer to that variable any more.

Answer 3

Here's what I found out about variable scopes:

my declarations are pretty clear and straightforward if used inside blocks. If used in main outside any block, they are a bit different though, meaning that a my variable declared outside a block is visible even inside functions called from anywhere inside the same file as long as these functions are defined within the same file. If declared inside a block, though, they are not visible to functions even if called from the same block. All my variables seem to live on the stack. And: you cannot localize them with local.

our variables live on the heap. Even if you have a my variable by the same name, the our variable can still be accessed through ${'var'}, which looks up a variable of that name in the symbol table and dereferences it. my variables, on the other hand, have not symbol table entries.

local variables seem to me like a relic from former Perl versions. They are just re-assignments to global (our) variables with block scope and resume their former values after the block terminates. I can see no real sense in using them.

My little program below shows all this, and it shows how badly a declared() test is missing, beyond the well-known defined() test, to identify undeclared variables as such.

 #!/usr/bin/perl

 use strict;

 ### This is about variable scoping with my, our and local
 my $fsv = "file scope";                 # visible for all code in this file
 our $gsv = "global scope";              # not different from my $fsv, except in packages
 our $lsv = "global";                    # global scope, but localized in subsequent block

 {
    my $bsv = "lex scope";               # visible only inside this block, not even in subs called from here
    $gsv = "visible everywhere";
    local $lsv = "global, but localized val";

    print "This is variable \$bsv with value $bsv inside block\n";
    print "This is variable \$fsv with value $fsv inside block\n";
    print "This is variable \$lsv with value $lsv inside block\n\n";
    print_vars("calledfromblock");
 }

 print_vars("calledfromoutside");


 no strict 'vars';                       # needed if testing variable for declaredness rather than definedness
 if ( defined $bsv ) {
    print "\$bsv as defined outside braces: $bsv\n"
 } else {
    print "\$bsv not defined outside braces\n";
 }
 print "This is variable \$lsv with value $lsv outside block\n";
 # use strict 'vars';                    # no strict 'vars' effective even in sub print_vars unless switched back on

 sub print_vars
 {
    my $whence = shift;
    my $gsv = "my variable";
    no strict 'refs';                    # needed to access the global var $gsv using ${'gsv'} despite the my declaration

    if ( $whence eq "calledfromblock" ) {
       print "\t print_vars called from within the block:\n";
       ( defined $bsv )     ? print "\$bsv is $bsv inside sub\n"     : print "\$bsv not defined inside sub\n";
       ( defined $fsv )     ? print "\$fsv is $fsv inside sub\n"     : print "\$fsv not defined inside sub\n";
       ( defined ${'gsv'} ) ? print "\$gsv is ${'gsv'} inside sub\n" : print "\$gsv not defined inside sub\n";
       ( defined ${'lsv'} ) ? print "\$lsv is ${'lsv'} inside sub\n" : print "\$lsv not defined inside sub\n";
    } else {
       print "\t print_vars called from outside the block:\n";
       ( defined $bsv ) ? print "\$bsv is $bsv inside sub\n" : print "\$bsv not defined inside sub\n";
       ( defined $fsv ) ? print "\$fsv is $fsv inside sub\n" : print "\$fsv not defined inside sub\n";
       ( defined $gsv ) ? print "\$gsv is $gsv inside sub\n" : print "\$gsv not defined inside sub\n";
       ( defined $lsv ) ? print "\$lsv is $lsv inside sub\n" : print "\$lsv not defined inside sub\n";
    }
    print "\n";
 }

Answer 4

###Example 1:

sub mess_with_foo {
    $foo=0;
}

sub myfunc {
    my $foo=20;
    mess_with_foo();
    print $foo;
}
myfunc();

###Example 2:

sub mess_with_foo {
    $foo=0;
}

sub myfunc {
    local $foo=20;
    mess_with_foo();
    print $foo;
}
myfunc();

Example 1 prints 20 because mess_with_foo() could not see my $foo. It could not change it. my $foo can only be seen in its scope of myfunc().

Example 2 prints 0 because mess_with_foo() can see my $foo and change it. local $foo can be seen in its scope of myfunc() AND in the scope of any function called from within its scope of myfunc().

That's the only difference. Neither my $foo nor local $foo will be seen outside of their scope of myfunc().