NAME
    constant::Atom - unique symbols

SYNOPSIS
            use constant::Atom qw (red yellow blue);
        
            my $color = red;
        
            print "Just as we thought!\n" if $color eq red;
            print "This will never happen.\n" if $color eq blue;
            print "Atoms never equal strings!\n" if $color eq 'red';
        
            print "Color is ".$color->name."\n";
        
            #The following raises an exception, because addition isn't defined for Atom objects.
            $color + 1; 

DESCRIPTION
    Unlike constants declared with `constant', Atoms are not associated with
    any specific scalar value. Instead, Atoms have their own independent
    identity, and will only compare positively (via the 'eq' test) with
    other identical Atoms. All other operations on Atoms are undefined,
    including casting to a string and casting to a number.

    Atoms are used in place of constants in situations where a unique value
    is needed to represent some idea or program state, but where that value
    is not naturally associated with a scalar value, and shouldn't be
    confused with one. Atoms are similar to C enums in this respect, except
    that Atoms do not have an ordinal value.

    Below is an example of where an Atom would solve a problem:

    # use constant::Atom 'error'; use constant 'error' => 999999;

            sub bar {
                my($arg) = @_;
            
                #Always return $arg for demonstration purposes (not error).
                return 1 ? $arg : error;
            }
        
            my $foo = bar(999999);
            print "Foo: $foo\n";
        
            print $foo eq error ? "Foo returned error." : "Foo returned $foo.";

    Output: Foo returned error.

    In the above example, the programmer is trying to choose some unlikely
    value to alias 'error' to. The problem is, if 'bar' is ever accidently
    called with this same value, the program will mistakenly believe that
    'error' had been returned.

    This doesn't happen with Atoms.

            use constant::Atom 'error';
    #       use constant 'error' => 999999;
        
            sub bar {
                my($arg) = @_;
            
                #Always return $arg for demonstration purposes (not error).
                return 1 ? $arg : error;
            }
        
            my $foo = bar(999999);
            print "Foo: $foo\n";
        
            print $foo eq error ? "Foo returned error." : "Foo returned $foo.";
    Output: Foo returned 999999.

COMPARISON TO ALTERNATIVES
    An alternative to using an Atom is to use a constant aliased to a
    reference to an arbitrary scalar:

            use constant myconstant => \"";

    There are two advantages of Atoms over this kind of constant

            - Scalar references can compare positively to numeric values:

                            use constant myconstant => \"";
                            my $numeric_value = myconstant + 0;
                            die "Trapped!" if $numeric_value == myconstant;
        
            - Atoms maintain their identity through serialization, even between processes:
                
                    use constant::Atom 'myatom';

                    use Storable qw (freeze thaw);
                    print "Just as we thought!" if myatom eq thaw(freeze(myatom));
                
ATOMS AS STRINGS
    An atom cast (stringified) into a string produces a representation that
    may be useful for debugging purposes:

            use constant::Atom 'myatom';
        
            my $value = myatom;
            print "Myatom cast into a string: $value\n";

    Output: Myatom cast into a string: Atom=SCALAR(0x18508dc)=main::myatom

    Stringified Atoms can be used as hash keys, matched to a regexps, etc.
    When this happens, the string value is not guaranteed to be unique.
    Although it is unlikely that you will ever accidently cast an Atom into
    a string, and even more unlikely that another string value will equal
    the string representation of the Atom, you might want to use
    constant::Atom::Strict to be 100% safe:

            use constant::Atom::Strict 'myatom';
        
            my $value = myatom;
            print "Myatom cast into a string: $value\n";

    Output: Can't cast Atom::Strict object 'main::myatom' into a string. Use
    the 'fullname' method for a string representation of this object at
    C:\test7.pl line 5

OTHER METHODS
    `name'
                package Languages;
                use constant::Atom qw (English);
                my $language = English;
        
                #Get the name of the constant
                print "Language: ".English->name."\n";
        
        Output: English

    `fullname'
                package Languages;
                use constant::Atom qw (English);
                my $language = English;
        
                #Get the string-representation of the constant, which is simply the fully-qualified symbol name.
                "$language" eq $language->fullname or die "These should be the same.";
        
        Output: Languages::English

SEE ALSO
    `constant'

REPOSITORY
    https://github.com/neilbowers/constant-Atom

AUTHOR
    This module was originally created by Jonathan R. Warden
    <john@newchester.com>, but is currently being maintained by Neil Bowers
    <neilb@cpan.org>.

COPYRIGHT
    Copyright 2004 Jonathan R. Warden. All rights reserved.

    This library is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself.