Perl - Packages & Modules

What are Packages?

The package statement switches the current naming context to a specified namespace (symbol table). Thus −

• A package is a collection of code which lives in its own namespace.
• A namespace is a named collection of unique variable names (also called a symbol table).
• Namespaces prevent variable name collisions between packages.
• Packages enable the construction of modules which, when used, won't clobber variables and functions outside of the modules's own namespace.
  
• The package stays in effect until either another package statement is invoked, or until the end of the current block or file.
• You can explicitly refer to variables within a package using the :: package qualifier.

Following is an example having main and Foo packages in a file. Here special variable __PACKAGE__ has been used to print the package name.

#!/usr/bin/perl

# This is main package
$i = 1; 
print "Package name : " , __PACKAGE__ , " $i\n"; 

package Foo;
# This is Foo package
$i = 10; 
print "Package name : " , __PACKAGE__ , " $i\n"; 

package main;
# This is again main package
$i = 100; 
print "Package name : " , __PACKAGE__ , " $i\n"; 
print "Package name : " , __PACKAGE__ ,  " $Foo::i\n"; 

1;

When above code is executed, it produces the following result −

Package name : main 1
Package name : Foo 10
Package name : main 100
Package name : main 10

BEGIN and END Blocks

You may define any number of code blocks named BEGIN and END, which act as constructors and destructors respectively.

BEGIN { ... }
END { ... }
BEGIN { ... }
END { ... }

• Every BEGIN block is executed after the perl script is loaded and compiled but before any other statement is executed.
• Every END block is executed just before the perl interpreter exits.
• The BEGIN and END blocks are particularly useful when creating Perl modules.

Following example shows its usage −

#!/usr/bin/perl

package Foo;
print "Begin and Block Demo\n";

BEGIN { 
    print "This is BEGIN Block\n" 
}

END { 
    print "This is END Block\n" 
}

1;

When above code is executed, it produces the following result −

This is BEGIN Block
Begin and Block Demo
This is END Block

What are Perl Modules?

A Perl module is a reusable package defined in a library file whose name is the same as the name of the package with a .pm as extension.
A Perl module file called Foo.pm might contain statements like this.

#!/usr/bin/perl

package Foo;
sub bar { 
   print "Hello $_[0]\n" 
}

sub blat { 
   print "World $_[0]\n" 
}
1;

Few important points about Perl modules

• The functions require and use will load a module.
• Both use the list of search paths in @INC to find the module.
• Both functions require and use call the eval function to process the code.
• The 1; at the bottom causes eval to evaluate to TRUE (and thus not fail).

The Require Function

A module can be loaded by calling the require function as follows −

#!/usr/bin/perl

require Foo;

Foo::bar( "a" );
Foo::blat( "b" );

You must have noticed that the subroutine names must be fully qualified to call them. It would be nice to enable the subroutine bar and blat to be imported into our own namespace so we wouldn't have to use the Foo:: qualifier.

The Use Function

A module can be loaded by calling the use function.

#!/usr/bin/perl

use Foo;

bar( "a" );
blat( "b" );

Notice that we didn't have to fully qualify the package's function names. The use function will export a list of symbols from a module given a few added statements inside a module.

require Exporter;
@ISA = qw(Exporter);

Then, provide a list of symbols (scalars, lists, hashes, subroutines, etc) by filling the list variable named @EXPORT: For Example −

package Module;

require Exporter;
@ISA = qw(Exporter);
@EXPORT = qw(bar blat);

sub bar { print "Hello $_[0]\n" }
sub blat { print "World $_[0]\n" }
sub splat { print "Not $_[0]\n" }  # Not exported!

1;

Create the Perl Module Tree

When you are ready to ship your Perl module, then there is standard way of creating a Perl Module Tree. This is done using h2xs utility. This utility comes alongwith Perl. Here is the syntax to use h2xs −

$h2xs -AX -n  ModuleName

For example, if your module is available in Person.pm file, then simply issue the following command −

$h2xs -AX -n Person

This will produce the following result −

Writing Person/lib/Person.pm
Writing Person/Makefile.PL
Writing Person/README
Writing Person/t/Person.t
Writing Person/Changes
Writing Person/MANIFEST

Here is the description of these options −

• -A omits the Autoloader code (best used by modules that define a large number of infrequently used subroutines)
• -X omits XS elements (eXternal Subroutine, where eXternal means external to Perl, i.e., C).
• -n specifies the name of the module.

So above command creates the following structure inside Person directory. Actual result is shown above.

• Changes
• Makefile.PL
• MANIFEST (contains the list of all files in the package)
• README
• t/ (test files)
• lib/ ( Actual source code goes here

So finally, you tar this directory structure into a file Person.tar.gz and you can ship it. You will have to update README file with the proper instructions. You can also provide some test examples files in t directory.

Installing Perl Module

Download a Perl module in the form tar.gz file. Use the following sequence to install any Perl Module Person.pm which has been downloaded in as Person.tar.gz file.

tar xvfz Person.tar.gz
cd Person
perl Makefile.PL
make
make install

The Perl interpreter has a list of directories in which it searches for modules (global array @INC).