Re: [NASL2] Metaphysical question on null values

On 8 Feb 2003, Michel Arboi wrote:

> The code about addition, substraction and array indexing was rather
> logical : adding or substracting anything to / from an undefined value
> is undefined, and accessing an array element with an undefined index
> is undefined too.
> However, all this is silent and can be badly surprising.
> So... Either we shout every time we try such an operation, or we
> consider those undefined values as 0, at least for + - and [ ]

IMHO, any use of an undefined value (unless explicitly allowed) is a
very good reason (at least) to print a warning--no sane program should
ever do such a thing. The only problem with this might be the amount of
superfluous warnings should the interpreter generate one whenever an
undefined value is used. This problem might be addresses in three
different ways:

1. the program is terminated
2. the result of the operation having one or more undefined arguments is
   a "secondary undefined value" and the interpreter does not generate
   any warnings unless it encounters a "primary" (e.g. non-secondary)
   undefined value
3. the undefined value is interpreted as 0 or "" (i.e. a neutral element
   for the operation in question) and the operation returns a normal
   result

Option 1 is likely to break to some scripts. Option 2 cannot be used for
conditional statements (if, while...). Option 3 might be tricky when an
overloaded operation is involved (is undefined + undefined == 0 or
""?)...or an operation having no natural neutral element (i.e. []!)--
the result should probably be undefined in these cases.

--Pavel Kankovsky aka Peak  [ Boycott Microsoft--http://www.vcnet.com/bms ]
"Resistance is futile. Open your source code and prepare for assimilation."