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From: =?ISO-8859-1?Q?Hibou57_=28Yannick_Duch=EAne=29?=
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Newsgroups: comp.lang.ada
Subject: Re: not null
Date: Thu, 5 Mar 2009 17:04:18 -0800 (PST)
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Date: 2009-03-05T17:04:18-08:00
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On 4 mar, 17:09, Adam Beneschan <a...@irvine.com> wrote:
> I'm having difficulty understanding the point. =A0You have to have the
> idea of the lack of a reference. =A0How else would you implement a
> linked list abstraction? =A0The first element has a reference to the
> second, and the second has a reference to the third, but the last one
> won't have a reference to anything. =A0This lack of a reference has to
> be represented somehow, right? =A0Suppose that access values in Ada were
> not allowed to have null values, and (for a linked list) you needed
> the concept of a value that may or may not refer to another object.
> You could write this as a variant record:
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 -- Adam

I use to think about it a long time ago and was thinking that the best
null or nil reference to an object would be a reference to a null or
nil object (a special singleton object). I do not mean an object which
will stand for all null references, rather an object for each type.

An other view : a null reference is a kind of boolean (it used this
way and cannot be used another way).... and with hindsight, this may
perhaps look funny. This boolean is a kind of constraint on a kind of
record with a discriminent (conceptually).

Just to talk about lists, this may be a circular list, with a flag on
the last element telling it is has a special meaning.