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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: not null
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Date: Thu, 5 Mar 2009 09:42:46 +0100
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Date: 2009-03-05T09:42:37+01:00
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On Wed, 4 Mar 2009 17:54:16 -0800 (PST), Adam Beneschan wrote:

> On Mar 4, 12:38 pm, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> wrote:
> 
>> Recursive types is not the single case where references are used. For smart
>> pointers not null constraint is very useful.
> 
> Right, I didn't say a "not null" constraint isn't useful; I'm just
> saying that it's also useful to have access objects that *can* be
> null, if needed.  Actually, I'm not clear on whether anyone disagrees
> with that.  This points to a curiosity in Hoare's abstract: first he
> implies that inventing "null references" was a big mistake, and then
> he says "declarations for non-null references" is the solution.  If
> allowing null references is a mistake, then the solution is to
> disallow them---not to allow the programmer to declare a reference
> that may not be null, right?

Right, this is what smart pointers use, not a constraint in the sense of an
Ada subtype, but a type that does not have null as a value. A smart pointer
is always valid.

> I think maybe Hoare misstated the
> problem.  The mistake wasn't inventing null references; the mistake
> was not providing a way for the programmer to disallow selected
> reference types or variables (objects) from being null.    This
> misstatement may have led to some confusion on this thread.

Yes, there are two cases: dynamically constrained references and statically
constrained ones. Both have their uses.

Concerning the invention of null, I can understand Hoare's point. There is
a comparable case with IEEE 754, where ideal values like NaN were invented.
Sometimes this adds a lot of pain, which could be spared. Sometimes it does
not.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de