Re: not null

[Brian Drummond <brian_drummond@btconnect.com>]

On Wed, 4 Mar 2009 17:47:03 -0800 (PST), Adam Beneschan <adam@irvine.com> wrote:

>On Mar 4, 5:32 pm, Brian Drummond <brian_drumm...@btconnect.com>
>wrote:
>> On Wed, 4 Mar 2009 08:09:56 -0800 (PST), Adam Beneschan <a...@irvine.com> wrote:

>> >I'm having difficulty understanding the point.  You have to have the
>> >idea of the lack of a reference.  How else would you implement a
>> >linked list abstraction?
>>
>> Possibly by pointing from the end back to the list head.
>>
>> It would make the "end of list" test more expensive (the head might have to be
>> kept in a register or passed around as another argument). I don't think it would
>> be pretty, but can't see any fundamental reason why it shouldn't work.
>
>Sure you could make it work, but what would be the point of doing so?
>And would it be any less error-prone?  Actually, it might even be
>moreso---when writing the operation to insert an element at the front
>of the list, you could forget to modify the reference in the tail.

I wasn't suggesting it was a good idea... 

Agree on all the above; and the error wouldn't bes ignificantly easier to find.

OTOH uninitialised (or incorrectly incremented) pointers in some languages cause
real grief... (my Algol W days are so far behind me, I can't remember if an
uninitialised reference was even possible)

>You could also make the "end of list" point to some special dummy
>object. 

I don't see that as semantically different from a null pointer.
Which perhaps just helps to make your point
>that the same set of mistakes applies as to a language that involves
>null reference, but the syntax of those mistakes is different, applies
>to both of these "solutions".

In which case, Hoare's point remains a mystery to me.

- Brian