NAME

Const::Exporter - Declare constants for export.

SYNOPSIS

Define a constants module:

  package MyApp::Constants;

  use Const::Fast;

  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/ fo $bar /]; # exported by default

and use that module:

  package MyApp;

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

  ...

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 ),
    ];

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 ];

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.

Using as part of a module with exported 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.

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.

AUTHOR

Robert Rothenberg, <rrwo at cpan.org>

LICENSE AND COPYRIGHT

Copyright 2014 Robert Rothenberg.

This program is free software; you can redistribute it and/or modify it under the terms of the the Artistic License (2.0). You may obtain a copy of the full license at:

http://www.perlfoundation.org/artistic_license_2_0

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