1 Writing Perl 5 Modules: An Introduction Keith Arner

2 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

3 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?)

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

5 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

6 How to use a Module Time::Local use Time::Local; $time = timelocal($sec, $min, $hours, $mday, $mon, $year); print "Seconds since epoch: $time\n";

7 Module Basics Learn by Example: MyRand.pm Refer to attached printout of MyRand.pm Note that line numbers are for reference purposes only

8 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

9 More on Package Names package MyRand is file MyRand.pm package Time is file Time.pm package Time::Local is file Time/Local.pm They have nothing to do with one another

10 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

11 and $VERSION have special tells Perl what functions you want to make available to the tells Perl how to export the functions (I won't go into the details) $VERSION is used to indicate compatibility

12 Exporting Functions Line 11 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 = ('srand', 'rand');

13 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)

14 A Function is a Function is a Function On lines 19 and 23, functions are declared and written exactly as they would be anywhere else

15 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

16 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' – –$^O & $/ –(3..99)

17 How do you use it? Make sure your module is in your include This can be augmented with -I or your PERL5LIB environment variable use MyRand; srand(5); print rand;

18 Advanced Topics Autoloading Selective Exporting Versioning Much, much more

19 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

20 More can export variables in addition to functions: = qw(rand srand and %EXPORT_TAGS can be used to export selectively

21 Selective Exporting It is not always desirable to export everything, is = qw (rand = qw ($seed); use MyRand qw(rand $seed);

22 Fancy Exporting Typing all the names of symbols to import can become a hassle, so %EXPORT_TAGS is = qw (rand = qw ($seed); %EXPORT_TAGS = ( funcs => [qw(rand srand)], vars => [qw($seed)] ); use MyRand qw(:funcs :vars);

23 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 ([^\$]+)/);

24 Much, much more Overriding builtin functions Cascading inheritance Automated Cleanup Object Oriented Programming

25 References Programming Perl, Second Edition. Wall, Christiansen & Schwartz –Chapters 5 and 7 Man pages –perlmod(1) Perl Cookbook. Christiansen & Torkington –Chapter 12

26 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