"package" Statement - Switching Name Space

This section provides a tutorial example on how to switch name space with the 'package' statement and how to access identifiers from other name space with double colon '::'.

Name Space: A compilation unit used to isolate identifiers from different parts of the source code. Name space is also called package. There are several basic rules about name spaces:

To verify those rules, I wrote the following program, NameSpaceTest.pl,

#- NameSpaceTest.pl
#- Copyright (c) 1995 by Dr. Herong Yang, http://www.herongyang.com/
#
   $j = "Java";                            # starting with main
   $f = "Fortran";
   $p = "Perl";
   $c = "C++";
   @list = ($j, $f, $p, $c);
   print("\nIn ",__PACKAGE__,"...\n");
   print("list = ",join(', ',@list),"\n"); 

   package Calendar;                       # entering Calendar
   $j = "January";
   $f = "February";
   $m = "March";
   $a = "April";
   @list = ($j, $f, $m, $a);
   print("\nIn ",__PACKAGE__,"...\n");
   print("list = ",join(', ',@list),"\n");

   {
      package Fruit;                       # entering Fruit
      $p = "peach";
      $o = "orange";
      $m = "mango";
      $a = "apple";
      @list = ($p, $o, $m, $a);
      print("\nIn ",__PACKAGE__,"...\n");
      print("list = ",join(', ',@list),"\n");
   }                                       # ending Fruit
                                           # back to Calendar
   print("\nBack in ",__PACKAGE__,"...\n");
   print("list = ",join(', ',@list),"\n");

   package Fruit;                          # entering Fruit
   print("\nBack in ",__PACKAGE__,"...\n");
   print("main::p = ",$main::p,"\n");
   print("Fruit::p = ",$Fruit::p,"\n"); 
   print("list = ",join(', ',@list),"\n");
   print("main::list = ",join(', ',@main::list),"\n");
   print("Calendar::list = ",join(', ',@Calendar::list),"\n");
   exit;

The output matches my expectation:

In main...
list = Java, Fortran, Perl, C++

In Calendar...
list = January, February, March, April

In Fruit...
list = peach, orange, mango, apple

Back in Calendar...
list = January, February, March, April

Back in Fruit...
main::p = Perl
Fruit::p = peach
list = peach, orange, mango, apple
main::list = Java, Fortran, Perl, C++
Calendar::list = January, February, March, April

Table of Contents

 About This Book

 Perl on Linux Systems

 ActivePerl on Windows Systems

 Data Types: Values and Variables

 Expressions, Operations and Simple Statements

 User Defined Subroutines

 Perl Built-in Debugger

Name Spaces and Perl Module Files

 Including Script Codes from Other Files

 do() Function - Including Script Files

 require() Function - Including Script Files

"package" Statement - Switching Name Space

 BEGIN(), CHECK(), INIT() and END() Functions

 Defining Your Own Perl Module

 CalendarModule.pm - A Sample Perl Module

 Symbolic (or Soft) References

 Hard References - Addresses of Memory Objects

 Objects (or References) and Classes (or Packages)

 Typeglob and Importing Identifiers from Other Packages

 String Built-in Functions and Performance

 File Handles and Data Input/Output

 Open Files in Binary Mode

 Open Directories and Read File Names

 File System Functions and Operations

 Converting Perl Script to Executable Binary

 Using DBM Database Files

 Using MySQL Database Server

 Socket Communication Over the Internet

 XML::Simple Module - XML Parser and Generator

 XML Communication Model

 SOAP::Lite - SOAP Server-Client Communication Module

 Perl Programs as IIS Server CGI Scripts

 CGI (Common Gateway Interface)

 XML-RPC - Remote Procedure Call with XML and HTTP

 RPC::XML - Perl Implementation of XML-RPC

 Integrating Perl with Apache Web Server

 CGI.pm Module for Building Web Pages

 LWP::UserAgent and Web Site Testing

 References

 PDF Printing Version