Packages

                    CHAPTER 19: PACKAGES


   The Problem

     - To support abstract data types (objects!!), a language must 
       provide a mechanism for a set of related subroutines to share
       variables and yet NOT allow these variables to be accessible
       by other subroutines

     - For example, a set of subroutines which implements a queue or a
       stack, needs to share some variables which must remain private
       to that set of subroutines

     - Obviously, global variables do not work here.  Nor do variables
       created by the local() function, since such variables are local
       to the enclosing block and can not be shared


   The Solution

     - PACKAGES!!


   Packages

     - A package provides an alternate namespace (symbol table) for 
       variables (and filehandles, formats, etc.)

     - The same variable name in two different packages refers to two
       different variables

     - The scope of a package declaration extends from the declaration
       itself to the end of the enclosing block


   The Main Package

     - The default package is called main and initially Perl begins
       compilation of all variables in the main namespace

     - So "global" variables are really variables in the main package


   Package Example

     - Here is a typical use of a package:

	 #!/usr/bin/perl

	 sub add_to_count
	 {
	   package Count;
	   $x = @_;
	   $count += $x;
	   print ("Count package: count is $count\n");
	 }
	   
	 sub subtract_from_count
	 {
	   package Count;
	   $x = @_;
	   $count -= $x;
	   print ("Count package: count is $count\n");
	 }
	   
	 $count = 17;
	 print ("Main package: count is $count\n");
	 &add_to_count (23);
	 print ("Main package: count is $count\n");
	 &subtract_from_count (13);
	 print ("Main package: count is $count\n");

       The results of this program are:

         Main package: count is 17
         Count package: count is 23
         Main package: count is 17
         Count package: count is 10
         Main package: count is 17

       Note that the $count variable in the main package and the $count
       in the Count package are two different variables.  But since
       the two subroutines are in the same Count package they share
       access to the Count package $count variable.


   More Package Advantages

      - No need to use local()

      - Can not inadvertently clobber variables outside the package

      - No danger of variable suicide in a subroutine
 

   Fully Qualified Variable Name 

      - You can override the private nature of package variables and
        refer to variables in other packages by using a fully qualified
        variable name

      - The fully qualified name is:
      
          package_name'variable_name

     - Ex.

         $main'count = 15;         # Set the $count variable in the main
                                   #   package to 15
         $Count'count = 89;        # Set the $count variable in the Count
                                   #   package to 89.  Can do this from
                                   #   ANY package!!