Re: not null

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Hyman Rosen <hyrosen@mail.com> writes:

> Georg Bauhaus wrote:
>> As said to have been seen on /.
>> another financial disaster, again caused
>> by making references to nothing, in Algol W,
>> http://qconlondon.com/london-2009/presentation/Null+References:+The+Billion+Dollar+Mistake
>
> You are misreading the abstract. First of all, it's not
> about some particular incident. He's talking about the
> overall impact.

Yes.  (I think Georg Bauhaus understood that.)

>... Second, he's not talking about failure
> to check for null references, he's talking about having
> null references at all, so Ada is no better.

There's nothing wrong with allowing programmers to use null pointers.

There's a lot wrong with giving programmers a null value for all
pointer types, whether they're wanted or not.  It's just a tripping
hazard.

Ada is a little bit better than languages that always allow null,
because it allows you to declare "not null" on an access [sub]type.
It would be even better if "not null" were the default -- ideally,
you should get a null value only if you ask for it explicitly.
It's too late to fix it now.

Ada is also better than languages that say deref of null leads to
unpredictable behavior.  At least you get a run-time error in Ada.
But it's still a tripping hazard.

See OCaml for a language that gets this right.

- Bob