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Chapter 8. Namespaces, libraries and modules

Contents

• Introduction

• Namespaces, visibility and scope of variables

• The art of separating namespaces

• Methods of modularization in Perl

• do statement

• Libraries and Keyword require

• Modules

• Standard modules
  • Processing dates
  • Time_in_Perl
• Summary
• Exercises

Introduction

The main problem in writing large programs is how separate parts of the code from each other and how to share variable between this parts (namespaces control). This is a critical part of programming language design and success of OO concept by-and-large was connected with built-in hierarchical namespaces model the OO provides.

Perl gradually added most of necessary mechanisms and as of version 5.10 has almost all the necessary goodies. Another positive feature is that Perl, unlike name other languages has an explicit concept of namespaces which make is suitable for writing very large programs. Namespaces in Perl are synonymous with a separate symbol tables that hold variables. They are the instrument to segregate code so that one piece of code does not conflict with another. In Perl new namespace is defined by package statement. It is active until the next package statement. If there is no package statement all variables belong to namespace main (global namespace).

Namespaces are names of separate symbol tables in compiler of interpreter and they let you segregate variables and functions into different boxes or compartments. They play the role similar to the role of directories in Unix system where full path for the file distinguishes it from the file with the same name but located in different directory. Namespaces hold only "global: names, my names that have lexical scope of a block are held separately.

A symbol table is a hash that holds all of the names defined in a namespace. All of the variable and function names belong to some symbol table (namespace). The hash for each namespace is named after the namespace with two colons. For example, the symbol table for the namespace utils is called %utils::. If you want to access symbol put_message from this namespace you need to write:

&utils::put_message("this is warning");

Here we call the function put_message from the namespace utils. In using Unix filesystem terms it would be a file put_message in the directory /utils while in Perl this is symbol put_message in the namespace utils.

In Perl new namespace is defined by package statement which is a compile time statement as it is influence the construction of symbol table which is compile-time activity. The main namespace in Perl is called main:: and is a default if no namespace statement exists. Variables from all other namespace require qualification with the namespace name if they are accessed outside the scope of package statement which defines this namespace. You can export variable from other named namespaces into main:: so that you can use them without qualification by the namespace name,

Each namespace with the exception of default (main::) is introduced by package statement. The scope of a package statement is from the statement to the next statement at the same level of nesting or the end of the innermost enclosing block. All identifiers located with the scope of package declaration belong to the namespace declared in the package statement. They will be placed in the symbol table with this name.

You can put a package declaration anywhere you can put a statement. Again, it is very important to undertan that this is a compile time statement. You can also switch back into previous namespace in more than one place; package statement merely determines which symbol table is used by the compiler for the rest of that block. That means that a given namespace can be used in multiple files.

Like Unix directories Perl namespaces are hierarchical and one namespace can be a part of another. The Unix filesystem's tremendous power and flexibility comes from the ability to put files in a multilevel directory tree and then locate them using the directory path. This makes it possible to access ten of thousands files belonging to operating system and various applications. Files with the same name but in different directories can coexist. For example:

/etc/hosts 
/etc/sysconfig/hosts

$etc::hosts
$stc::sysconfig::hosts

Like directory can store any type of file, namespaces can store any type of variables: scalar, arrays, hashes, functions, etc. The digram '::' serves as the separator between herarchically organized namespaces, just as '/' is the separator in the UNIX world. There are two basic ways to specify the namespace to which a variable belongs:

• either directly access it using fully qualified name of the variable
• can use the package keyword to access it.

Global::execute();

$Mail::message;

print keys(%my::important::addresses);

You can use the variable "%namespace::" to get a list of all the symbols declared in a given namespace. For example:

foreach $key (keys %Time::) {
   print "\$$key => $Time::{$key}\n"
   print "\@$key => @$Time::{$key}\n";
   print "\%$key => @{[%$Time::{$key}\n"]}\n"; 
}

In addition to namespaces Perl provides three options for segregating code into independent semi-isolated parts:

• Libraries are similar to shell dot operation and just include the code "in-place" into the program at execution time. As such they provide only physical separation of code in a separate file. They share the namespace with the module into which they included so only local variables with subroutines provide some namespace separation. This is a run -time operation, not at compile time like in #include statement C. They were introduced in Perl 4. To include a library one need to use require statement.
• Packages. Package statement is the way Perl introduced the namespace concept into the language. It has scope and all variables and subroutines within this scope belong to the package. The main difference with the libraries is that each package has its own namespace. You can export certain names into local namespace that uses the package using export statement. This provides an excellent control of namespaces.
• Modules is a specially organized package located a separate file. They can be included into scripts at compile time via statement use Module. From this point they are closer to C #include statements. Each module use its own namespace which corresponds to the name of the module. They were introduced in Perl 5. Simplifying, module is a collection of subroutines and variables, all belonging to the same package and stored away in its own file. While subroutines allow us to bundle up individual tasks, modules are more about bundling up an entire area of activity similarly to OO concept of classes, but without inheritance.

If we want to get perl to read that file and use it as part of our own program, we have three ways of doing this:

• do
• require
• use

Use is now dominant form and all standard modules are included via the use statement.

@INC and %INC are built-in variables that define the list of directories were library or module will be searched for loading with use or require statements

Review of literature

Perl documentation has man page for package statement.

perlmod -- Perl modules (packages and symbol tables), perlmodlib describes Perl stndard module

In two recommended free books

• Simon Cozin book chapter on references is pretty weak
• David Medinets Perl 5 by Example Ebook chapter on references can serve as a good reference. It is not a tutorial (see Perl 5 by Example/ chapter 8: References)

Useful articles are:

• Namespaces: Coping with Scoping

Some useful documents and informative posts are available at the Web sites www.perl.com and PerlMonks

Etc

Society

Groupthink : Two Party System as Polyarchy : Corruption of Regulators : Bureaucracies : Understanding Micromanagers and Control Freaks : Toxic Managers :   Harvard Mafia : Diplomatic Communication : Surviving a Bad Performance Review : Insufficient Retirement Funds as Immanent Problem of Neoliberal Regime : PseudoScience : Who Rules America : Neoliberalism  : The Iron Law of Oligarchy : Libertarian Philosophy

Quotes

War and Peace : Skeptical Finance : John Kenneth Galbraith :Talleyrand : Oscar Wilde : Otto Von Bismarck : Keynes : George Carlin : Skeptics : Propaganda  : SE quotes : Language Design and Programming Quotes : Random IT-related quotes :  Somerset Maugham : Marcus Aurelius : Kurt Vonnegut : Eric Hoffer : Winston Churchill : Napoleon Bonaparte : Ambrose Bierce :  Bernard Shaw : Mark Twain Quotes

Bulletin:

Vol 25, No.12 (December, 2013) Rational Fools vs. Efficient Crooks The efficient markets hypothesis : Political Skeptic Bulletin, 2013 : Unemployment Bulletin, 2010 :  Vol 23, No.10 (October, 2011) An observation about corporate security departments : Slightly Skeptical Euromaydan Chronicles, June 2014 : Greenspan legacy bulletin, 2008 : Vol 25, No.10 (October, 2013) Cryptolocker Trojan (Win32/Crilock.A) : Vol 25, No.08 (August, 2013) Cloud providers as intelligence collection hubs : Financial Humor Bulletin, 2010 : Inequality Bulletin, 2009 : Financial Humor Bulletin, 2008 : Copyleft Problems Bulletin, 2004 : Financial Humor Bulletin, 2011 : Energy Bulletin, 2010 : Malware Protection Bulletin, 2010 : Vol 26, No.1 (January, 2013) Object-Oriented Cult : Political Skeptic Bulletin, 2011 : Vol 23, No.11 (November, 2011) Softpanorama classification of sysadmin horror stories : Vol 25, No.05 (May, 2013) Corporate bullshit as a communication method  : Vol 25, No.06 (June, 2013) A Note on the Relationship of Brooks Law and Conway Law

History:

Fifty glorious years (1950-2000): the triumph of the US computer engineering : Donald Knuth : TAoCP and its Influence of Computer Science : Richard Stallman : Linus Torvalds  : Larry Wall  : John K. Ousterhout : CTSS : Multix OS Unix History : Unix shell history : VI editor : History of pipes concept : Solaris : MS DOS :  Programming Languages History : PL/1 : Simula 67 : C : History of GCC development :  Scripting Languages : Perl history   : OS History : Mail : DNS : SSH : CPU Instruction Sets : SPARC systems 1987-2006 : Norton Commander : Norton Utilities : Norton Ghost : Frontpage history : Malware Defense History : GNU Screen : OSS early history

Classic books:

The Peter Principle : Parkinson Law : 1984 : The Mythical Man-Month :  How to Solve It by George Polya : The Art of Computer Programming : The Elements of Programming Style : The Unix Hater’s Handbook : The Jargon file : The True Believer : Programming Pearls : The Good Soldier Svejk : The Power Elite

Most popular humor pages:

Manifest of the Softpanorama IT Slacker Society : Ten Commandments of the IT Slackers Society : Computer Humor Collection : BSD Logo Story : The Cuckoo's Egg : IT Slang : C++ Humor : ARE YOU A BBS ADDICT? : The Perl Purity Test : Object oriented programmers of all nations : Financial Humor : Financial Humor Bulletin, 2008 : Financial Humor Bulletin, 2010 : The Most Comprehensive Collection of Editor-related Humor : Programming Language Humor : Goldman Sachs related humor : Greenspan humor : C Humor : Scripting Humor : Real Programmers Humor : Web Humor : GPL-related Humor : OFM Humor : Politically Incorrect Humor : IDS Humor : "Linux Sucks" Humor : Russian Musical Humor : Best Russian Programmer Humor : Microsoft plans to buy Catholic Church : Richard Stallman Related Humor : Admin Humor : Perl-related Humor : Linus Torvalds Related humor : PseudoScience Related Humor : Networking Humor : Shell Humor : Financial Humor Bulletin, 2011 : Financial Humor Bulletin, 2012 : Financial Humor Bulletin, 2013 : Java Humor : Software Engineering Humor : Sun Solaris Related Humor : Education Humor : IBM Humor : Assembler-related Humor : VIM Humor : Computer Viruses Humor : Bright tomorrow is rescheduled to a day after tomorrow : Classic Computer Humor

The Last but not Least Technology is dominated by two types of people: those who understand what they do not manage and those who manage what they do not understand ~Archibald Putt. Ph.D

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Last modified: March 12, 2019