=head1 TITLE

New 'tristate' pragma to allow undef to take on NULL semantics

=head1 VERSION

  Maintainer: Nathan Wiger <nate@wiger.org>
  Date: 23 Sep 2000
  Last Modified: 29 Sep 2000
  Mailing List: perl6-language@perl.org
  Number: 275
  Version: 2
  Status: Frozen

=head1 ABSTRACT

RFC 263 proposed the introduction of a C<null> keyword for introducting
tristate logic into Perl 6. However, that was abandoned in favor of the
approach specified here, a C<tristate> pragma.

=head1 DESCRIPTION

The C<tristate> pragma allows for undef to take on the RDBMS concept of
C<NULL>, in particular:

   1. Any operation between a NULL and any other value results
      in NULL

   2. Any comparison between a NULL and any other value is false

   3. No NULL value is equal to any other NULL

   4. A NULL value is neither defined nor undefined

The C<tristate> pragma is lexically scoped, so that it obeys code
blocks:

   $a = undef;
   $b = 1;
   $c = $a + $b;    # 1
   {
      use tristate;
      $d = $a + $b; # undef
   }
   $e = $c + $d;    # 1

In addition, the C<defined> builtin is overloaded as an "is not null"
operator under the pragma:

   use tristate;
   $name = undef;
   die "Badness" unless defined $name;  # $name is not null

This simplifies C<undef>, actually: C<defined($x)> will always return
false if C<$x> is C<undef>, regardless of the C<use tristate> pragma.

For more details on theoretical issues, please see the references or RFC
263.

=head1 IMPLEMENTATION

No idea, too burned out.

=head1 MIGRATION

None, unless some fool has a custom C<tristate> module that they
wrote to navigate the tristate area of New York, New Jersey, and
Connecticut. But that should be C<Tristate> anyways.

=head1 REFERENCES

RFC 263: Add null() keyword and fundamental data type

http://www.sitelite.nl/mysql/manual_Problems.html#IDX666

http://www.unb.ca/web/transpo/mynet/mtx19.htm#r2