Author image Neil Bowers


Devel::Constants - translates constants back to named symbols


  # must precede use constant
  use Devel::Constants 'flag_to_names';
  use constant A => 1;
  use constant B => 2;
  use constant C => 4;
  my $flag = A | B;
  print "Flag is: ", join(' and ', flag_to_names($flag) ), "\n";


Declaring constants is very convenient for writing programs, but as Perl often inlines them, retrieving their symbolic names can be tricky. This worse with lowlevel modules that use constants for bit-twiddling.

Devel::Constants makes this much more manageable.

It silently wraps around the constant module, intercepting all constant declarations. It builds a hash, associating the values to their names, from which you can retrieve their names as necessary.

Note that you must use Devel::Constants before constant, or the magic will not work and you will be very disappointed. This is very important, and if you ignore this warning, the authors will feel free to laugh at you (at least a little.

By default, Devel::Constants only intercept constant declarations within the same package that used the module. Also by default, it stores the constants for a package within a private (read, otherwise inaccessible) variable. You can override both of these.

Passing the package flag to Devel::Constants with a valid package name will make the module intercept all constants subsequently declared within that package. For example, in the main package you might say:

  use Devel::Constants package => NetPacket::TCP;
  use NetPacket::TCP;

All of the TCP flags declared within NetPacket::TCP are now available.

It is also possible to pass in a hash reference in which to store the constant values and names:

  my %constant_map;
  use Devel::Constants \%constant_map;
  use constant NAME     => 1;
  use constant RANK     => 2;
  use constant SERIAL   => 4;
  print join(' ', values %constant_map), "\n";


By default, Devel::Constants exports no subroutines. You can import its two helper functions optionally by passing them on the use line:

  use Devel::Constants qw( flag_to_names to_name );
  use constant FOO => 1;
  use constant BAR => 2;
  print flag_to_names(2);
  print to_name(1);

You may also import these functions with different names, if necessary. Pass the alternate name after the function name. Beware that this is the most fragile of all options. If you do not pass a name, Devel::Constants may become confused:

  # good
  use Devel::Constants
    flag_to_names => 'resolve',
  # WILL WORK IN SPITE OF POOR FORM (the author thinks he's clever)
  use Devel::Constants
    flag_to_names => 'resolve';

  use Devel::Constants
    package => WD::Kudra;

Passing the import flag will import any requested functions into the named package. This is occasionally helpful, but it will overwrite any existing functions in the named package. Be a good neighbor:

  use Devel::Constants
    import => 'my::other::namespace',

Note that constant also exports subroutines, by design.


flag_to_names($flag, [ $package ])

This function resolves a flag into its component named bits. This is generally only useful for known bitwise flags that are combinations of named constants. It can be very handy though. $flag is the flag to decompose. The function does not modify it. The $package parameter is optional. If provided, it will use flags set in another package. In the NetPacket::TCP example above, you can use it to find the symbolic names of TCP packets, such as SYN or RST set on a NetPacket::TCP object.

to_name($value, [ $package ])

This function resolves a value into its constant name. This does not mean that the value necessarily comes from the constant, but merely that it has the same value as the constant. (For example, 2 could be the result of a mathematical operation, or it could be a sign to dump core and bail out. to_name only guarantees the same value, not the same semantics. See PSI::ESP if this is not acceptable.) As with flag_to_names, the optional $package parameter will look for constants declared in a package other than the current.


  • figure out a better way to handle flag_to_names (inefficient algorithm)

  • allow potential capture lists?

  • sync up better with allowed constant names in constant

  • evil nasty Damianesque idea: locally redefining constants


chromatic chromatic at wgz dot org, with thanks to "Benedict" at for the germ of the idea (

Thanks also to Tim Potter and Stephanie Wehner for NetPacket::TCP.

Version 1.01 released by Neil Bowers <neilb at cpan dot org>.


Copyright (c) 2001, 2005 chromatic. Some rights reserved.

This is free software. You may use, modify, and distribute it under the same terms as Perl 5.8.x itself.