Intro Modules

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Introduction to Perl modules

Simon Whitaker

([email protected])

May 2003


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Introduction to Perl modules (SW) 2

Evolving towards modularity

In its simplest form, programming in a

language like Perl involves writing lines of codethat execute sequentially:

$name = "Alice";

print "Hello $name!\n";

# prints Hello Alice!


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Writing line-by-line code works for simple tasks,

but it can get unnecessarily repetitive:

$name = "Alice";

print "Hello $name!\n";

$name2 = "Bob";

print "Hello $name2!\n";

$name3 = "Charlie";

print "Hello $name3!\n";


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Problems arise when I need to revise my code.

For example, imagine that I need to translatemy program into French. I have to alter 3 linesof code:

$name = "Alice";

print "Bonjour $name!\n";

$name2 = "Bob";

print "Bonjour $name2!\n";$name3 = "Charlie";

print "Bonjour $name3!\n";


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Better to encapsulate the repetitive stuff in a

subroutine, which I only have to amend once:

sub greet {

print "Hello $_[0]\n";

}

$name = "Alice";

greet($name);$name2 = "Bob";

greet($name2);


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And in French...

sub greet {

print "Bonjour $_[0]\n";

}

$name = "Alice";

greet($name);

$name2 = "Bob";greet($name2);


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Evolving towards modularityI can now use greet() anywhere in my

program, making my code easier to maintain.

However, what if I want to use greet() in

other programs? I can copy it into eachprogram, but then I have the same problem ofhaving to maintain multiple instances of thesame code.

This is where modules come in.


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What is a module?

A module is a library of Perl code that can be

included in your Perl program.

When you include a Perl module in a program,

the functionality of that module is available foryou to use inside your own program.


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Where do modules come from?

Perl modules come from a variety of sources:

Standard modules: modules that are installedwhen you install Perl

CPAN: the Comprehensive Perl Archive

Network a global network of Perl modulerepositories

Third parties: e.g. in-house code libraries,Bioperl, etc


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How to use a module

To include a module in your program, use Perl's

use keyword. The general form is:

use Modu l e ;

Typically, the module will export its mostpopular subroutines and variables into yourprogram. You can then use these subroutines

and variables just as if they were declared inyour program.


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How to use a module

For example:

use Hello; # use the Hello module

greet("Alice"); # greet() is exported

# by the Hello module


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How to use a module

To use subroutines or variables that aren't

exported by default, precede their name withthe module name followed by a double colon:

use Hello;

Hello::greet("Alice");

# calls greet() from the Hello module


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How to use a module

Alternatively, you can state explicitly which

variables and subroutines you want to exportwhen you load the module, using the generalform:

us e Modul e qw( $v ar i ab l e s ubr out i ne ) ;

where $variable and subroutine are the

things you want to export. This form overridesthe default form only the variables andsubroutines you specify are imported.


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Example 1: Text::WrapText::Wrap is a simple but very useful module.

It's part of the standard module library, whichmeans that it's installed when you install Perl.

Text::Wrap contains a subroutine called

wrap() (exported by default), which wrapstext to form neat paragraphs.

wrap() takes three arguments: the indentstring for the first line of a paragraph, theindent string for subsequent lines, and the textto be wrapped.


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Example 1: Text::Wrapuse Text::Wrap;

$indent_first = " ";

$indent_subsq = "";

$text = "For the life of me I couldnever understand why Mr Perkins didn'tenjoy his job. He had everything a mancould want: a place on the board, anabsent boss and a cat named Henry.";

print wrap($indent_first,$indent_subsq, $text);


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Example 1: Text::Wrap

Output:

For the life of me I could

never understand why Mr Perkins

didn't enjoy his job. He had

everything a man could want: a

place on the board, an absent

boss and a cat named Henry.


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Text::Wrap also has a variable called

$columns, which determines how manycolumns my text wraps to. Unlikewrap(),$columns isn't exported from Text::Wrap by

default, so I refer to it here using its fully

qualified name:

# start of program as before

$Text::Wrap::columns = 20;print wrap($indent_first,

$indent_subsq, $text);


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Output:

For the life ofme I could never

understand why Mr

Perkins didn't

enjoy his job. He

had everything a

man could want: a

place on the board,an absent boss and

a cat named Henry.


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3 benefits of using modules

1. Saves time and effort

2. Greater portability3. Modularity


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Benefit 1: Saves time and effort

There are thousands of Perl modules out there,

covering a huge range of common (and not socommon) tasks. Don't re-invent the wheel!

If you need to grab data from a web page,create PDF files on the fly or connect to adatabase from your Perl program, I have greatnews for you someone else has already done

the hard work. Better still, they'll share it withyou!


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Benefit 2: Greater portability

Perl modules are generally written with

portability in mind. Again let someone else dothe hard work!

For example: imagine that you want to move afile from within your Perl program. Perl doesn'thave a built-in move function, so you might betempted to write:

`mv oldfile newfile`;


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Benefit 2: Greater portability

This works fine on UNIX. But if you try to run

the program on Windows, or another operatingsystem that doesn't have amv command, it

fails.

This, however, is portable across OSs:

use File::Copy;

move("oldfile", "newfile");


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Benefit 3: Modularity

As the name suggests, modules are modular!

With modules you can parcel bits of code thatdo specific tasks (e.g. wrapping text) into

discrete bundles, which can be re-usedelsewhere.

If the code needs to be updated you only needto change the module, not dozens of Perlprograms.

b f d b b


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A brief word about objects

From time to time you'll come across object-

oriented (OO) Perl modules.

It's not important at this stage to understand

much about OO programming or how it works,but it helps to be familiar with the syntax ituses so that you feel comfortable using OO Perlmodules.

b f d b b


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A brief word about objects

Objects represent complex entities, such as

people, books or DNA sequences. Generally,you create objects using new:

$alice = new Student();

and interact with them using methods, whichwork just like subroutines:

print $alice->gradeAverage();

E l 2 N FTP


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Example 2: Net::FTP

Net::FTP is an object-oriented Perl module that

allows you to access FTP servers. As usual, youstart by importing the module:

use Net::FTP;

Then you create a new Net::FTP object:

$ftp = new Net::FTP("ftp.mysite.com");

E l 2 N FTP


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Example 2: Net::FTP

Now you can interact with your new object:

$ftp->login("username", "password");

$ftp->cwd("/pub");

$ftp->get("mrperkins.doc");

$ftp->quit();

M d l d t ti


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Module documentationWhen you install Perl modules, you get access

to documentation for that module.

To read the documentation for a module, usethe perldoc command:

$ perldoc File::Copy

Alternatively, on Unix you can use the mancommand. On Windows you will have HTMLdocumentation available there's a link in theStart menu.

F th di


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Further reading

For the official low-down on modules:

$ perldoc perlmod

For a list of standard modules:

$ perldoc perlmodlib

To read in the bath: Learning Perl Objects,References & Modules by Randal L. Schwartzwith Tom Phoenix. June 2003 (est.)

S


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Summary

Modules are libraries of code that you can

insert into your Perl programs Modules save you time and effort, by

providing easy access to common tasks

Use the perldoc command to getdocumentation on a Perl module

Search CPAN for general-purpose modules

Documents

Intro Modules