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Writing Perl 5 Modules:An Introduction

Keith Arner

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Outline

• What do we mean by Modules and Classes?

• Module Basics– Using Modules, Packages, Special Variables,

Exporting Functions

• Advanced Concepts– Autoloading, More Exporting, Versioning

• Ask questions at any time

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Topics Not Covered

• Objects

• Modules available on CPAN

• Writing modules for CPAN distribution

• Each of these would make a good topic for a future talk (Any volunteers?)

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Modules and Classes

• There are two types of modularization under Perl 5– Modules : library functions– Classes : object-oriented programming

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What is a Module?

• A module is a .pm file that defines a library of related functions

• Modules are conceptually similar to old-fashioned Perl libraries (.pl files), but have a cleaner implementation– selective namespace cluttering– simpler function invocation

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How to use a Module Time::Local

• use Time::Local;

$time = timelocal($sec, $min, $hours, $mday, $mon,

$year);

print "Seconds since epoch: $time\n";

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Module Basics

• Learn by Example: MyRand.pm• Refer to attached printout of MyRand.pm• Note that line numbers are for reference

purposes only

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Package Names and Filenames

• Package name is declared on line 1

• This should be the same as the filename, without the .pm extension

• If it is different, your functions will not be exported correctly

• Should begin with a capital letter to avoid possible conflict with pragmas

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More on Package Names

• package MyRandis file MyRand.pm

• package Timeis file Time.pm

• package Time::Localis file Time/Local.pm

• They have nothing to do with one another

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The Package: Your Own Private Namespace

• Each package is a separate namespace for functions and variables

• There is no collision of names between namespaces

• $seed in the package main and $seed in the package MyRand are distinct variables

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Magic Variables

• @EXPORT, @ISA and $VERSION have special meanings

• @EXPORT tells Perl what functions you want to make available to the public

• @ISA tells Perl how to export the functions (I won't go into the details)

• $VERSION is used to indicate compatibility

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Exporting Functions

• Line 11 sets @EXPORT to contain a list of the names of functions to be exported

• Note that we use qw() to generate the list– This is just syntactic sugar for:@EXPORT = ('srand', 'rand');

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What does it mean to Export?

• Your functions are defined in your own package

• Exporting makes the functions available in another package as if they were defined in the other package

• This is done by fiddling with symbol tables (which is a topic for another talk)

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A Function is a Functionis a Function

• On lines 19 and 23, functions are declared and written exactly as they would be anywhere else

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Module Specific State

• Line 15 declares a variable local to the module that can maintain it's state between function calls

• $seed is visible to all functions in this file• $seed does not conflict with $seed in

any other module, nor in a mainline program

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Declaring Victory

• All Perl modules must return a true value to indicate that they completed successfully

• Line 35 shows the canonical return• Any true value will do:

– 'true'– 3.1415926535– $^O & $/– (3..99)

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How do you use it?

• Make sure your module is in your include path, @INC

• This can be augmented with -I or your PERL5LIB environment variable

• use MyRand;srand(5);print rand;

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Advanced Topics

• Autoloading

• Selective Exporting

• Versioning

• Much, much more

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Autoloading

• The autoloader is used to improve performance of large modules (such as POSIX.pm)

• Functions are split into separate files and compiled only as needed

• AutoSplit.pm is used to split modules

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More Exporting

• @EXPORT can export variables in addition to functions:– @EXPORT = qw(rand srand $seed)

• @EXPORT_OK and %EXPORT_TAGS can be used to export selectively

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Selective Exporting

• It is not always desirable to export everything, so @EXPORT_OK is used

• @EXPORT = qw (rand srand);@EXPORT_OK = qw ($seed);

• use MyRand qw(rand $seed);

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Fancy Exporting

• Typing all the names of symbols to import can become a hassle, so %EXPORT_TAGS is used

• @EXPORT = qw (rand srand);@EXPORT_OK = qw ($seed);%EXPORT_TAGS = (

funcs => [qw(rand srand)],vars => [qw($seed)] );

• use MyRand qw(:funcs :vars);

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Versioning

• $VERSION is used to indicate the version of the module

• use MyRand 2.0;– will fail if $MyRand::VERSION < 2.0

• Can be used with RCS Keywords:– $VERSION = ('$REVISION 1.0$' =~

m/Revision ([^\$]+)/);

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Much, much more

• Overriding builtin functions

• Cascading inheritance with @ISA• Automated Cleanup

• Object Oriented Programming

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References• Programming Perl, Second Edition.

Wall, Christiansen & Schwartz– Chapters 5 and 7

• Man pages– perlmod(1)

• Perl Cookbook. Christiansen & Torkington– Chapter 12

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Summary

• Modules are libraries of functions

• A simple module just exports a set of functions

• Perl modules can be expanded in many directions for arbitrarily sophisticated libraries