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Packages in packages in Perl

In perl, all your variables (except "my" ones, which we'll leave for another day) are arranged into packages, and by default that's a package called "main". So if you write about a scalar called $jeremy, that's really $main::jeremy and if you write about a named block of code called &mary, that's really &main::mary.

Why?

So that, as your programs grow, you can arrange all the variables and pieces of code into their own uniquely named modules. That then allows you to use the same name several times over without a conflict.

Good news - but it doesn't go far enough so far; what I've described only goes one level further and module files (for example) would all end up in a single massive "pancake" directory. So the :: notation can be nested, and package namespaces can be nested within package namespaces. As far as file organisation is concerned, you'll be working with a directory structure. This sort of thing is very much used on the CPAN, from where you can download modules such as Net::FTP - to be found once installed in a file called FTP.pm in the Net directory.

Here's an example program and some modules that show this in action.

$ perl pk2lev
tutor
designer
beauty therapist
$

And the pk2lev program:

use ellis::graham;
use ellis::lisa;
use whelan::lisa;
print ellis::graham::job();
print ellis::lisa::job();
print whelan::lisa::job();

The directory structure:

$ ls ellis whelan pk2lev
pk2lev

ellis:
graham.pm lisa.pm

whelan:
lisa.pm
$

And a sample module file:

$ cat ellis/lisa.pm

package ellis::lisa;
sub job {
return "designer\n";
}

1;
$
(written 2005-12-16, updated 2006-06-09)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P209 - Subroutines in Perl
  [3833] Learning to use existing classes in Perl - (2012-08-10)
  [3574] Perl functions such as chop change their input parameters - (2012-01-10)
  [3066] Separating groups of variables into namespaces - (2010-11-24)
  [2929] Passing a variable number of parameters in to a function / method - (2010-08-20)
  [2833] Fresh Perl Teaching Examples - part 2 of 3 - (2010-06-27)
  [2550] Do not copy and paste code - there are much better ways - (2009-12-26)
  [2069] Efficient calls to subs in Perl - avoid duplication, gain speed - (2009-03-07)
  [1921] Romeo and Julie - (2008-12-04)
  [1860] Seven new intermediate Perl examples - (2008-10-30)
  [1850] Daisy the Cow and a Pint of Ginger Beer - (2008-10-21)
  [1826] Perl - Subs, Chop v Chomp, => v , - (2008-10-08)
  [1784] Global - Tcl, PHP, Python - (2008-09-03)
  [1782] Calling procs in Tcl and how it compares to Perl - (2008-09-02)
  [1202] Returning multiple values from a function (Perl, PHP, Python) - (2007-05-24)
  [1163] A better alternative to cutting and pasting code - (2007-04-26)
  [969] Perl - $_ and @_ - (2006-12-07)
  [775] Do not duplicate your code - (2006-06-23)
  [588] Changing @INC - where Perl loads its modules - (2006-02-02)
  [357] Where do Perl modules load from - (2005-06-24)
  [308] Call by name v call by value - (2005-05-11)
  [96] Variable Scope - (2004-10-22)

P218 - Perl - More Objects
  [4366] Changing what operators do on objects - a comparison across different programming languages - (2014-12-26)
  [4356] Object factories in C++, Python, PHP and Perl - (2014-12-19)
  [4098] Using object orientation for non-physical objects - (2013-05-22)
  [4096] Perl design patterns example - (2013-05-20)
  [3941] Building an object based on another object in Perl - (2012-12-03)
  [3581] Perl - calls to methods that use => - what do they mean? - (2012-01-16)
  [3377] What do I mean when I add things in Perl? - (2011-08-02)
  [3098] Learning Object Orientation in Perl through bananas and perhaps Moose - (2010-12-21)
  [3097] Making Perl class definitions more conventional and shorter - (2010-12-20)
  [2972] Some more advanced Perl examples from a recent course - (2010-09-27)
  [2876] Different perl examples - some corners I rarely explore - (2010-07-18)
  [2811] Igloos melt in the summer, but houses do not - (2010-06-15)
  [2717] The Multiple Inheritance Conundrum, interfaces and mixins - (2010-04-11)
  [2651] Calculation within objects - early, last minute, or cached? - (2010-02-26)
  [2427] Operator overloading - redefining addition and other Perl tricks - (2009-09-27)
  [1949] Nuclear Physics comes to our web site - (2008-12-17)
  [1819] Calling base class constructors - (2008-10-03)
  [1665] Factory method example - Perl - (2008-06-04)
  [1664] Example of OO in Perl - (2008-06-03)
  [1435] Object Oriented Programming in Perl - Course - (2007-11-18)
  [1320] Perl for Larger Projects - Object Oriented Perl - (2007-08-25)
  [1217] What are factory and singleton classes? - (2007-06-04)
  [930] -> , >= and => in Perl - (2006-11-18)
  [831] Comparison of Object Oriented Philosophy - Python, Java, C++, Perl - (2006-08-13)
  [656] Think about your design even if you don't use full UML - (2006-03-24)
  [592] NOT Gone phishing - (2006-02-05)
  [246] When to bless a Perl variable - (2005-03-15)
  [227] Bellringing and Programming and Objects and Perl - (2005-02-25)

P616 - Perl - FTP and Telnet Modules
  [975] Answering ALL the delegate's Perl questions - (2006-12-09)


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