Exporting and Importing Package Identifiers

Perl Tutorials - Herong's Tutorial Examples - Version 5.32, by Dr. Herong Yang

Perl Tutorials - Herong's Tutorial Examples

∟Typeglob and Importing Identifiers from Other Packages

∟Exporting and Importing Package Identifiers

This section provides a tutorial example on how to access identifiers from another package as aliases, *identifier = *package::identifier.

Instead of asking all the calling packages to define aliases, Perl offers a solution to help package to export identifiers (symbols) automatically to the calling packages. Perl offers a helping package called Exporter, which can be used to help your package to define aliases into the calling packages.

To prepare a package to export identifiers (symbols), you need to add the following lines:

   package ModuleName;
   require Exporter;
   @ISA = qw(Exporter);
   @EXPORT = qw(...);       # symbols to export by default
   @EXPORT_OK = qw(...);    # symbols to export on request

To import identifiers (symbols) from an included package, you need to use the "use" statement in one of the following formats:

  use ModuleName;           # import default symbols into my package
  use ModuleName qw(...);   # import listed symbols into my package
  use ModuleName ();        # do not import any symbols

Let's see how I improved my old package to use Exporter. Here is the calendar package ready to export identifier:

#- CalendarExported.pm
#- Copyright (c) 1999 by Dr. Herong Yang, http://www.herongyang.com/
#
   package CalendarExported;
   require Exporter;
   @ISA = qw(Exporter);
   @EXPORT = qw($sec $min $hour $mday $mon $year $wday $yday);
   @EXPORT_OK = qw($swday $smon $smday $stime $syear &isLeapYear);
   sub BEGIN {
      $author = "Dr. Herong Yang";
      ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$x) = localtime();
      ($swday,$smon,$smday,$stime,$syear) = split(' ',localtime());
   }
   sub isLeapYear {
      local $day59 = time() - ($yday-59)*24*60*60;
      local ($0,$1,$2,$3,$m,$5,$6,$7,$8) = localtime($day59);
      $m = 0 unless $m==1;
      return $m;
   }
1;

Here is a program to show you how to import identifiers with the "use" statements:

#- CalendarImportTest.pl
#- Copyright (c) 1999 by Dr. Herong Yang, http://www.herongyang.com/
#
   use CalendarExported qw($smon $smday $stime $syear &isLeapYear);
   print("Date: $smon $smday $syear\n");
   print("Leap year? ",&isLeapYear(),"\n");
   exit;

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

Symbolic (or Soft) References

Hard References - Addresses of Memory Objects

Objects (or References) and Classes (or Packages)