TITLE

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

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

ABSTRACT

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

DESCRIPTION

The tristate pragma allows for undef to take on the RDBMS concept of 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 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 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 undef, actually: defined($x) will always return false if $x is undef, regardless of the use tristate pragma.

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

IMPLEMENTATION

No idea, too burned out.

MIGRATION

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

REFERENCES

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