Tom Christiansen wrote:
>
> >Currently, Perl has the concept of C<undef>, which means that a value is
> >not defined. One thing it lacks, however, is the concept of C<null>,
> >which means that a value is known to be unknown or not applicable. These
> >are two separate concepts.
>
> No, they aren't.
Uhhh, yes, they are...
undef null
------------------------ ------------------------
$a = undef; $a = null;
$b = 1; $b = 1;
$c = $a + b; $c = $a + $b;
$c is 1 $c is null
The keyword C<null> means that a value is B<known to be unknown>. This
means a couple important things, with semantics far different from
C<undef>:
1. Any math or string operation between a null and
non-null value results in null
2. No null value is equal to any other null, unlike
undef
3. A null value is neither defined nor undefined
To recap: There is no 1:1 mapping between C<undef> and <null>. Any
attempt to do so is inaccurate. For more details, please read the
references.
...
=head1 REFERENCES
http://www.sitelite.nl/mysql/manual_Problems.html#IDX666
http://www.unb.ca/web/transpo/mynet/mtx19.htm#r2
