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# Writing PERL Modules

## What are Packages?

- 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 clobbber variables and functions outside of the modules's own namespace

## The Package Statement

- package statement switches the current naming context to a specified namespace (symbol table)

- If the named package does not exists, a new namespace is first created.

    $i = 1; print "$i\n"; # Prints "1"
    package foo;
    $i = 2; print "$i\n"; # Prints "2"
    package main;
    print "$i\n"; # Prints "1"

- The package stays in effect until either another package statement is invoked, or until the end of the end of the current block or file.

- You can explicitly refer to variables within a package using the :: package qualifier

    $PACKAGE_NAME::VARIABLE_NAME

    For Example:
    $i = 1; print "$i\n"; # Prints "1"
    package foo;
    $i = 2; print "$i\n"; # Prints "2"
    package main;
    print "$i\n"; # Prints "1"

    print "$foo::i\n"; # Prints "2"

## 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.

# 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 on the end).

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 noteable points about modules

- The functions require and use will load a module.

- Both use the list of search paths in @INC to find the module (you may modify it!)

- Both 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

    #!/usr/bin/perl

    require Foo;

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

Notice above that the subroutine names must be fully qualified (because they are isolated in their own package)

It would be nice to enable the functions 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  Module Name

    # For example, if your module is available in Person.pm file
    $h2xs -AX -n Person

    This will produce 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 descritpion 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 and you can ship it. You would have to update README file with the proper instructions. You can provide some test examples files in t directory.

# Installing Perl Module

Installing a Perl Module is very easy. Use the following sequence to install any Perl Module.

    perl Makefile.PL
    make
    make install

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

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