News flash: I'm going to Australia in February, so this Web site may be switched off at any time.
When you've declared a variable but before you've ever put anything in it, that variable is said to have the null value. If you try to perform computations on a null value, your program will terminate abnormally. This protects you from logic errors caused by using a variable that has never been defined.
You can test to see whether a variable is undefined (that is,
whether it has a null value) by using the ``/'' and
``\'' operators:
/ e'' succeeds and returns the
value of e if e is undefined; if e has
a value, it fails.
\ e'' succeeds and returns the
value of e if e has a value; if e is null,
it fails.
There is a special predefined variable, ``&null'',
so this expression
foo := &null
causes the variable foo to become undefined; any old value
it might have had is discarded.
john@nmt.edu