NAME

Const::Exporter - Declare constants for export.

VERSION

version v1.2.3

SYNOPSIS

Define a constants module:

package MyApp::Constants;

our $zoo => 1234;

use Const::Exporter

   tag_a => [                  # use MyApp::Constants /:tag_a/;
      'foo'  => 1,             # exports "foo"
      '$bar' => 2,             # exports "$bar"
      '@baz' => [qw/ a b c /], # exports "@baz"
      '%bo'  => { a => 1 },    # exports "%bo"
   ],

   tag_b => [                  # use MyApp::Constants /:tag_b/;
      'foo',                   # exports "foo" (same as from ":tag_a")
      '$zoo',                  # exports "$zoo" (as defined above)
   ];

# `use Const::Exporter` can be specified multiple times

use Const::Exporter

   tag_b => [                 # we can add symbols to ":tab_b"
      'moo' => $bar,          # exports "moo" (same value as "$bar")
   ],

   enums => [

     [qw/ goo gab gub /] => 0, # exports enumerated symbols, from 0..2

   ],

   default => [qw/ foo $bar /]; # exported by default

and use that module:

package MyApp;

use MyApp::Constants qw/ $zoo :tag_a /;

...

Dynamically Creating Constants

You may also import a predefined hash of constants for exporting dynamically:

use Const::Exporter;

my %myconstants = (
       'foo'  => 1,
       '$bar' => 2,
       '@baz' => [qw/ a b c /],
       '%bo'  => { a => 1 },
);

# ... do stuff

Const::Exporter->import(
     constants => [%myconstants],        # define constants for exporting
     default   => [ keys %myconstants ], # export everything in %myconstants by default
);

DESCRIPTION

This module allows you to declare constants that can be exported to other modules.

To declare constants, simply group then into export tags:

package MyApp::Constants;

use Const::Exporter

  tag_a => [
     'foo' => 1,
     'bar' => 2,
  ],

  tag_b => [
     'baz' => 3,
     'bar',
  ],

  default => [
     'foo',
  ];

Constants in the default tag are exported by default (that is, they are added to the @EXPORTS array).

When a constant is already defined in a previous tag, then no value is specified for it. (For example, bar in tab_b above.) If you do give a value, Const::Exporter will assume it's another symbol.

Your module can include multiple calls to use Const::Exporter, so that you can reference constants in other expressions, e.g.

use Const::Exporter

  tag => [
      '$zero' => 0,
  ];

use Const::Exporter

  tag => [
      '$one' => 1 + $zero,
  ];

or even something more complex:

use Const::Exporter

   http_ports => [
      'HTTP'     => 80,
      'HTTP_ALT' => 8080,
      'HTTPS'    => 443,
   ];

use Const::Exporter

   http_ports => [
      '@HTTP_PORTS' => [ HTTP, HTTP_ALT, HTTPS ],
   ];

Constants can include traditional constant symbols, as well as scalars, arrays or hashes.

Constants can include values defined elsewhere in the code, e.g.

our $foo;

BEGIN {
   $foo = calculate_value_for_constant();
}

use Const::Exporter

  tag => [ '$foo' ];

Note that this will make the symbol read-only. You don't need to explicitly declare it as such.

Enumerated constants are also supported:

use Const::Exporter

  tag => [

    [qw/ foo bar baz /] => 1,

  ];

will define the symbols foo (1), bar (2) and baz (3).

You can also specify a list of numbers, if you want to skip values:

use Const::Exporter

  tag => [

    [qw/ foo bar baz /] => [1, 4],

  ];

will define the symbols foo (1), bar (4) and baz (5).

You can even specify string values:

use Const::Exporter

  tag => [

    [qw/ foo bar baz /] => [qw/ feh meh neh /],

  ];

however, this is equivalent to

use Const::Exporter

  tag => [
    'foo' => 'feh',
    'bar' => 'meh',
    'baz' => 'neh',
  ];

Objects are also supported,

use Const::Exporter

 tag => [
   '$foo' => Something->new( 123 ),
 ];

Export Tags

By default, all symbols are exportable (in @EXPORT_OK.)

The :default tag is the same as not specifying any exports.

The :all tag exports all symbols.

KNOWN ISSUES

Support for older Perl versions

This module requires Perl v5.10 or newer.

Pull requests to support older versions of Perl are welcome. See "SOURCE".

Exporting Functions

Const::Exporter is not intended for use with modules that also export functions.

There are workarounds that you can use, such as getting Const::Exporter to export your functions, or munging @EXPORT etc. separately, but these are not supported and changes in the future my break our code.

Mixing POD with Tags

The following code is a syntax error, at least with some versions of Perl:

use Const::Exporter

=head2 a

=cut

  a => [ foo => 1 ],

=head2 b

=cut

  b => [ bar => 2 ];

If you want to mix POD with your declarations, use multiple use lines, e.g.

=head2 a

=cut

use Const::Exporter
  a => [ foo => 1 ];

=head2 b

=cut

use Const::Exporter
  b => [ bar => 2 ];

SUPPORT FOR OLDER PERL VERSIONS

This module requires Perl v5.10.1 or later.

Future releases may only support Perl versions released in the last ten years.

SEE ALSO

See Exporter for a discussion of export tags.

Similar Modules

• Exporter::Constants

  This module only allows you to declare function symbol constants, akin to the constant module, without tags.

• Constant::Exporter

  This module only allows you to declare function symbol constants, akin to the constant module, although you can specify tags.

• Constant::Export::Lazy

  This module only allows you to declare function symbol constants, akin to the constant module by defining functions that are only called as needed. The interface is rather complex.

• Const::Fast::Exporter

  This module will export all constants declared in the package's namespace.

SOURCE

The development version is on github at https://github.com/robrwo/Const-Exporter and may be cloned from git://github.com/robrwo/Const-Exporter.git

BUGS

Please report any bugs or feature requests on the bugtracker website https://github.com/robrwo/Const-Exporter/issues

When submitting a bug or request, please include a test-file or a patch to an existing test-file that illustrates the bug or desired feature.

AUTHOR

Robert Rothenberg rrwo@cpan.org

CONTRIBUTORS

• Slaven Rezić slaven@rezic.de
• B. Estrade estrabd@cpan.org

COPYRIGHT AND LICENSE

This software is Copyright (c) 2014-2023 by Robert Rothenberg.

This is free software, licensed under:

The Artistic License 2.0 (GPL Compatible)