Perl module: Difference between revisions

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[[File:Perl_module.png|thumb|Diagram of the mechanism of using perl modules.]]

A '''Perl module''' is a discrete component of [[software]] for the [[Perl]] [[programming language]]. Technically, it is a particular set of [[Convention (norm)|conventions]] for using [[#Perl packages and namespaces|Perl's package mechanism]] that has become universally adopted.{{discuss|Packages and modules}}

A module defines its [[source code]] to be in a ''package'' (much like a [[Java package]]), the Perl mechanism for defining [[namespaces]], e.g. ''CGI'' or ''Net::FTP'' or ''XML::Parser''; the file structure mirrors the [[namespace]] structure (e.g. the [[source code]] for ''Net::FTP'' is in ''Net/FTP.pm''). Furthermore, a module is the Perl equivalent of the [[class (computer science)|class]] when [[object-oriented programming]] is employed.{{discuss|Packages and modules}}

A collection of modules, with accompanying [[Software documentation|documentation]], [[Build automation|build scripts]], and usually a [[test suite]], composes a '''distribution'''. The Perl community has a sizable library of distributions available for search and download via [[CPAN]].

Perl is a language allowing many different styles of programming. You'reA developer is as likely to find a module written in a [[Procedural programming|procedural]] style (for example, [https://metacpan.org/module/Test::Simple Test::Simple]) as [[Object-oriented programming|object-oriented]] (e.g. [https://metacpan.org/module/XML::Parser XML::Parser]), both are considered equally valid according to what the module needs to do. Modules might also be used to [[mixin]] methods ([https://metacpan.org/module/DBIx::Class DBIx::Class]) or be a [[compiler directive|pragma]] ([http://perldoc.perl.org/strict.html strict.pm]) which has an effect immediately upon being loaded. Modules can even be used to alter the syntax of the language. The effect of Perl modules are usually limited to the current [[scope (programming)|scope]] in which it was loaded.

It is common for Perl modules to have embedded documentation in Perl's [[Plain Old Documentation]] format. POD imposes little structure on the author. It is flexible enough to be used to write articles, web pages and even entire books such as [https://web.archive.org/web/20070316052422/http://www.oreilly.com/catalog/pperl3/colophon.html Programming Perl]. Contrast with [[javadoc]] which is specialized to documenting [[Java (programming language)|Java]] classes. By convention, module documentation typically follows the structure of a [[Manual page (Unix)|Unix man page]].

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What follows are examples of "[[Hello world program|Hello, World]]" implemented in different styles of modules. It must be understood that a module is not necessary in Perl; functions and code can be defined and used anywhere. This is just for example purposes. Contrast with [[Java (programming language)|Java]] where a class is always necessary. A real "Hello, World" function would be written like so:

<sourcesyntaxhighlight lang="perl">

sub hello { "Hello, world!\n" }

print hello();

</syntaxhighlight>

</source>

or simply printed in one line:

<sourcesyntaxhighlight lang="perl">

print "Hello, world!\n";

</syntaxhighlight>

</source>

===Procedural example===

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====''hello_world.pl''====

<sourcesyntaxhighlight lang="perl">

#!/usr/bin/env perl

# Loads the module and imports any functions into our namespace

# (defaults to "main") exported by the module. Hello::World exports

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print hello(); # prints "Hello, world!\n"

print hello("Milky Way"); # prints "Hello, Milky Way!\n"

</syntaxhighlight>

</source>

====''Hello/World.pm''====

<sourcesyntaxhighlight lang="perl">

# "package" is the namespace where the module's functionality/data resides.

# It dictates the name of the file if you want it to be "use"d.

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# this is rarely used and it would instead die() (exit with an error).

1;

</syntaxhighlight>

</source>

----

Since Hello/World.pm is not in your @INC path, you must specify . on the command line to run the above example:

perl -I. hello_world.pl

===Object-oriented example===

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====''hello_world.pl''====

<sourcesyntaxhighlight lang="perl">

#!/usr/bin/env perl

use Hello::World;

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$greeting->print; # prints "Hello, Pittsburgh!\n"

$hello->print; # still prints "Hello, Milky Way!\n"

</syntaxhighlight>

</source>

====''Hello/World.pm''====

<sourcesyntaxhighlight lang="perl">

# In Perl there is no special 'class' definition. A namespace is a class.

package Hello::World;

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sub new {

my($class, %args) = @_;

my $self = bless({}, $class);

my $target = exists $args{target} ? $args{target} : "world";

$self->{target} = $target;

return $self;

}

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sub target {

my $self = shift;

if ( @_ ) {

my $target = shift;

$self->{target} = $target;

}

return $self->{target};

}

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sub to_string {

my $self = shift;

return "Hello, $self->{target}!";

}

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sub print {

my $self = shift;

print $self->to_string(), "\n";

}

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1;

</syntaxhighlight>

</source>

----

==Perl packages and namespaces==

A running Perl program has a built-in [[namespace]] called "<code>main</code>", which is the default name. For example, a subroutine called <code>Sub1</code> can be called as <code>Sub1()</code> or <code>main::Sub1()</code>. With a variable the appropriate [[Sigil (computer programming)|sigil]] is placed in front of the namespace; so a scalar variable called <code>$var1</code> can also be referred to as <code>$main::var1</code>, or even <code>$::var1</code>. Other namespaces can be created at any time.

<sourcesyntaxhighlight lang="perl">

package Namespace1;

$var1 = 1; # created in namespace Namespace1, which is also created if not pre-existing

our $var2 = 2; # also created in that namespace; our required if use strict is applied

my $var3 = 3; # lexically-scoped my-declared - NOT in any namespace, not even main

</syntaxhighlight>

</source>

<sourcesyntaxhighlight lang="perl">

$Namespace2::var1 = 10; # created in namespace Namespace2, also created if not pre-existing

our $Namespace2::var2 = 20; # also created in that namespace

my $Namespace2::var3 = 30;#compilation error:my-declared variables CAN'T belong to a package

</syntaxhighlight>

</source>

Package declarations apply package scope till the next package declaration or the end of the block in which the declaration is made.

<sourcesyntaxhighlight lang="perl">

our $mainVar = 'a';

package Sp1;

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$mainVar = 'b';

{

# NOTE previously created packages and package variables still accessible

package Sp1;

our $sp1bVar = 'bb';

print "$main::mainVar\t$sp1aVar\t$sp1bVar\n"; # note mainVar needs qualifying

{

{

package Sp2;

our $sp2bVar = 'bbb';

print "$main::mainVar\t$Sp1::sp1aVar$Sp1::sp1bVar\t$sp2aVar$sp2bVar\n";

} # note mainVar and sp1...Var need qualifying

print "$main::mainVar\t$sp1bVar$sp1aVar\t$Sp2::sp2bVar$Sp2::sp2aVar\n";

} # note package Sp1 applies by default

# main applies again by default; all package variables still accessible as long as qualified

print "$mainVar\t$Sp1::sp1aVar$Sp2::sp2bVar\n";

</syntaxhighlight>

</source>

===Packages and modules===

Conventionally, namespaces are associated with modules; in practice, there is usually one namespace per module and vice versa, but that's not mandated by the language. For example, the 'standard' module CGI.pm has the following declaration at its top:

<sourcesyntaxhighlight lang="perl">

package CGI;

</syntaxhighlight>

</source>

This module, and its functionality, would commonly be invoked as follows:

<sourcesyntaxhighlight lang="perl">

use CGI (':standard'); # imports many functions, including b()

...

print b('Hello, world'); # outputs <b>Hello, world</b>

</syntaxhighlight>

</source>

A 'missing' subroutine ''could'' be added from the using program's namespace.

<sourcesyntaxhighlight lang="perl">

sub CGI::bi { # define target namespace (CGI) and sub name (bi)

return b(i($_[0]));

}

</syntaxhighlight>

</source>

and invoked as below:

<sourcesyntaxhighlight lang="perl">

print CGI::bi('Hello, world'); # outputs <b><i>Hello, world</i></b>

</syntaxhighlight>

</source>

However, though technically feasible, that would be dubious programming practice. You might just as well define the sub in the calling namespace, and call it from that namespace.