Constant::Generate - Common tasks for symbolic constants


Simplest use

  use Constant::Generate [ qw(CONST_FOO CONST_BAR) ];
  printf( "FOO=%d, BAR=%d\n", CONST_FOO, CONST_BAR );


  use Constant::Generate [qw(ANNOYING STRONG LAZY)], type => 'bits';
  my $state = (ANNOYING|LAZY);
  $state & STRONG == 0;

With reverse mapping:

  use Constant::Generate
    type => "bits",
    mapname => "client_type_to_str";
  my $client_type = CLIENT_IRSSI | CLIENT_PURPLE;
  print client_type_to_str($client_type); #prints 'CLIENT_IRSSI|CLIENT_PURPLE';

Generate reverse maps, but do not generate values. also, push to exporter

  #Must define @EXPORT_OK and tags beforehand
  our @EXPORT_OK;
  use Constant::Generate {
    O_RDONLY => 00,
    O_WRONLY => 01,
    O_RDWR       => 02,
    O_CREAT  => 0100
  }, tag => "openflags", type => 'bits';
  my $oflags = O_RDWR|O_CREAT;
  print openflags_to_str($oflags); #prints 'O_RDWR|O_CREAT';

DWIM Constants

  use Constant::Generate {
    RDONLY  => 00,
    WRONLY  => 01,
    RDWR    => 02,
    CREAT   => 0100
  }, prefix => 'O_', dualvar => 1;
  my $oflags = O_RDWR|O_CREAT;
  O_RDWR eq 'RDWR';

Export to other packages

  package My::Constants
  BEGIN { $INC{'My/} = 1; }
  use base qw(Exporter);
  use Constant::Generate [qw(FOO BAR BAZ)],
        tag => "my_constants",
        export_ok => 1;
  package My::User;
  use My::Constants qw(:my_constants);
  FOO == 0 && BAR == 1 && BAZ == 2 &&
        my_constants_to_str(FOO eq 'FOO') && my_constants_to_str(BAR eq 'BAR') &&
        my_constants_to_str(BAZ eq 'BAZ');


Constant::Generate provides useful utilities for handling, debugging, and generating opaque, 'magic-cookie' type constants as well as value-significant constants.

Using its simplest interface, it will generate a simple enumeration of names passed to it on import.

Read import options to use.


All options and configuration for this module are specified at import time.

The canonical usage of this module is

  use Constant::Generate $symspec, %options;

Symbol Specifications

This is passed as the first argument to import and can exist as a reference to either a hash or an array. In the case of an array reference, the array will just contain symbol names whose values will be automatically assigned in order, with the first symbol being 0 and each subsequent symbol incrementing on the value of the previous. The default starting value can be modified using the start_at option (see "Options").

If the symbol specification is a hashref, then keys are symbol names and values are the symbol values, similar to what constant uses.

By default, symbols are assumed to correlate to a single independent integer value, and any reverse mapping performed will only ever map a symbol value to a single symbol name.

For bitflags, it is possible to specify type => 'bits' in the "Options" which will modify the auto-generation of the constants as well as provide suitable output for reverse mapping functions.

Basic Options

The second argument to the import function is a hash of options.

All options may be prefixed by a dash (-option) or in their 'bare' form (option).


This specifies the type of constant used in the enumeration for the first argument as well as the generation of reverse mapping functions. Valid values are ones matching the regular expression /bit/i for bitfield values, and ones matching /int/i for simple integer values.

You can also specify /str/i for string constants. When the symbol specification is an array, the value for the string constants will be the strings themselves.

If type is not specified, it defaults to integer values.


Only valid for auto-generated numeric values. This specifies the starting value for the first constant of the enumeration. If the enumeration is a bitfield, then the value is a factor by which to left-shift 1, thus

  use Constant::Generate [qw(OPT_FOO OPT_BAR)], type => "bits";
  OPT_FOO == 1 << 0;
  OPT_BAR == 1 << 1;
  #are true

and so on.

For non-bitfield values, this is simply a counter:

  use Constant::Generate [qw(CONST_FOO CONST_BAR)], start_at => 42;
  CONST_FOO == 42;
  CONST_BAR == 43;

Specify a tag to use for the enumeration.

This tag is used to generate the reverse mapping function, and is also the key under which symbols will be exported via %EXPORT_TAGS.


Specify the name of the reverse mapping function for the enumeration. If this is omitted, it will default to the form

  $tag . "_to_str";

where $tag is the "tag" option passed. If neither are specified, then a reverse mapping function will not be generated.

export, export_ok, export_tags

This group of options specifies the usage and modification of @EXPORT, @EXPORT_OK and %EXPORT_TAGS respectively, which are used by Exporter.

Values for these options should either be simple scalar booleans, or reference objects corresponding to the appropriate variables.

If references are not used as values for these options, Constant::Generate will expect you to have defined these modules already, and otherwise die.


Set this to a string to be prefixed to all constant names declared in the symbol specification; thus the following are equivalent:

  use Constant::Generate [qw( MY_FOO MY_BAR MY_BAZ )];

With auto-prefixing:

  use Constant::Generate [qw( FOO BAR BAZ )], prefix => "MY_";

When prefixes are specified, the default is that reverse mapping functions will display only the 'bare', user-specified name. Thus:

  use Constant::Generate [qw( FOO )], prefix => "MY_", mapname => "const_str";
  const_str(MY_FOO) eq 'FOO';

Setting show_prefix to a true value will display the full name.

Dual-Var Constants

Use of dual variable constants (which return an integer or string value depending on the context) can be enabled by passing stringy_vars to Constant::Generate, or using Constant::Generate::Dualvar:


This will apply some trickery to the values returned by the constant symbols.

Normally, constant symbols will return only their numeric value, and a reverse mapping function is needed to retrieve the original symbolic name.

When dualvar is set to a true value the values returned by the constant subroutine will do the right thing in string and numeric contexts; thus:

  use Constant::Generate::Dualvar [qw(FOO BAR)];
  FOO eq 'FOO';
  FOO == 0;

The "show_prefix" option controls whether the prefix is part of the stringified form.

Do not rely too much on dualvar to magically convert any number into some meaningful string form. In particular, it will only work on scalars which are directly descended from the constant symbols. Paritcularly, this means that unpack()ing or receiving data from a different process will not result in these special stringy variables.

The stringy_vars option is an alias for dualvar, which is supported for backwards compatibility.


The following options enable constant subroutines which return lists of the symbols or their values:

  use Constant::Generate [qw(
  allvalues => "VALS",
  allsyms => "SYMS";
  printf "VALUES: %s\n", join(", ", VALUES);
  # => 0, 1, 2 (in no particular order)
  printf "SYMBOLS: %s\n", join(", ", SYMS);
  # => FOO, BAR, BAZ (in no particular order)

Or something potentially more useful:

  use Constant::Generate [qw(
  type => 'bits',
  allvalues => 'symptoms',
  mapname => "symptom_str";
  my $remedies = {
    COUGH, "Take some honey",
    SNEEZE, "Buy some tissues",
    HICCUP, "Drink some water"
  my $patient = SNEEZE | COUGH | ZOMBIES;
  foreach my $symptom (symptoms()) {
    next unless $patient & $symptom;
    my $remedy = $remedies->{$symptom};
    if(!$remedy) {
      printf "Uh-Oh, we don't have a remedy for %s. Go to a hospital!\n",
    } else {
      printf "You should: %s\n", $remedy;

Sometimes it is convenient to have a list of all the constants defined in the enumeration. Setting allvalues will make Constant::Generate create a like-named constant subroutine which will return a list of all the values created.


Like "allvalues", but will return a list of strings for the constants in the enumeration.


This module also allows you to define a 'constants' module of your own, from which you can export constants to other files in your package. Figuring out the right exporter parameters is quite hairy, and the export options can natually be a bit tricky.

In order to succesfully export symbols made by this module, you must specify either export_ok or export as hash options to import. These correspond to the like-named variables documented by Exporter.

Additionally, export tags can be specified only if one of the export flags is set to true (again, following the behavior of Exporter). The auto-export feature is merely one of syntactical convenience, but these three forms are effectively equivalent:

Nicest way:

  use base qw(Exporter);
  use Constant::Generate
    [qw(FOO BAR BAZ)],
    export => 1,
    tag => "some_constants"

A bit more explicit:

  use base qw(Exporter);
  use Constant::Generate
    [qw(FOO BAR BAZ)],
      export => \our @EXPORT,
      export_tags => \our %EXPORT_TAGS,
      tag => "some_constants",
      mapname => "some_constants_to_str",


  use base qw(Exporter);
  our @EXPORT;
  my @SYMS;
    @SYMS = qw(FOO BAR BAZ);
  use Constant::Generate \@SYMS, mapname => "some_constants_to_str";
  push @EXPORT, @SYMS, "some_constants_to_str";
  $EXPORT_TAGS{'some_constants'} = [@SYMS, "some_constants_to_str"];

Also note that any "allvalues", "allsyms", or "mapname" subroutines will be exported according to whatever specifications were configured for the constants themselves.


The dualvar or stringy_var option can be short-handed by doing the following:

  use Constant::Generate::Dualvar [qw(
  )], prefix => 'MY_';
  MY_FOO eq 'FOO';



It's somewhat ironic that a module which aims to promote the use of symbolic constants has all of its configuration options determined by hashes and strings.



Copyright (c) 2011 by M. Nunberg

You may use and distribute this software under the same terms and conditions as Perl itself, OR under the terms and conditions of the GNU GPL, version 2 or greater.