Writing Perl 5 Modules: An Introduction

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Writing Perl 5 ModulesAn 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
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• 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 TimeLocal

• use TimeLocaltime 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 MyRand is file MyRand.pm
• package Time is file Time.pm
• package TimeLocal is 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

• _at_EXPORT, _at_ISA and VERSION have special meanings
• _at_EXPORT tells Perl what functions you want to
  make available to the public
• _at_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 _at_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_at_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,
  _at_INC
• This can be augmented with -I or your PERL5LIB
  environment variable
• use MyRandsrand(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

• _at_EXPORT can export variables in addition to
  functions
• _at_EXPORT qw(rand srand seed)
• _at_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 _at_EXPORT_OK is used
• _at_EXPORT qw (rand srand)_at_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
• _at_EXPORT qw (rand srand)_at_EXPORT_OK qw
  (seed)EXPORT_TAGS ( funcs gt qw(rand
  srand), vars gt 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 MyRandVERSION lt 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 _at_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