Class 03: Perl Program Design

Held Wednesday, January 24, 2001

Summary

Today we consider issues in the design of Perl programs, including documentation, packages, modules, and objects.

Notes

Overview

• CGI Script Presentations
  • What did they do?
  • Particular problems?
  • Important lessons?
• Writing better Perl
  • Practice, Practice, Practice
  • Packages and Modules
  • Objects

Documentation

• Learn and use pod.
• See chapter 16 of Programming Perl.

Modules and Packages

• Ways of grouping stuff.
  • Packages provide separate namespaces (so that you an share a variable between many procedures but not automatically between all procedures).
  • Modules are "the fundamental unit of code reuse": A group of procedures and variables that naturally work together.
• Modules and packages are typically grouped together.
• A module is simply a package stored in a file of the form PackageName.pm.
  • Custom dictates that modules begin with a capital letter.
• Typical format (modified from p. 302 of Programming Perl, 3rd edition)

  package PackageName;
  require Exporter;
  
  our @ISA = qw(Exporter); # Says we can export symbols
  our @EXPORT = qw(symbols); # Says we export these symbols
                             # by default           
  our @EXPORT_OK = qw(symbols); # Says we can export these
                                # symbols by request
  our $VERSION = version-number;
  
  variables-and-procedures;
  
  1; # Ends the package
  

• Getting access to parts of a module
  • use PackageName;
    Pollute your namespace with default symbols
  • use Packagename(symbols);
    Pollute your namespace with selected symbols
  • See p. 303 for more variations
• Also possible to use Module::thing

Objects

• Some object-oriented stuff (e.g., inheritance, polymorphism), some syntactic sugar.
• Normal calling format:

  object->method(params);
  

• Essentially sugar for

  ObjectModule::method(object, params);
  

  • Unless I have it wrong.
• An object is essentially a reference to a hash that has been blessed with a module it belongs to (its class).