Re: RM and abstract syntax trees

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Wed, 31 Oct 2007 11:55:44 -0300
Message-ID: <47289773$0$14864$9a566e8b@news.aliant.net>

Marshall wrote:

> On Oct 31, 5:31 am, David BL <davi..._at_iinet.net.au> wrote:
>

>>On Oct 31, 7:23 pm, "David Cressey" <cresse..._at_verizon.net> wrote:
>>
>>
>>>In this whole discussion,  I have a big problem understanding what is meant
>>>by "the equivalent of pointers".  A pointer is NOT an arbitrary meaningless
>>>identifier.  A pointer is an address. If you assign an arbitrary meaningless
>>>identifier to an object for reference purposes,  that is NOT the same thing
>>>as referencing the object via a pointer.
>>
>>>If you need the concept of arbitrary meaningless identifier in order to make
>>>a point about how RM represents trees,  go ahead.  Just don't call them
>>>"pointers" and don't assert that they are the quivalent of pointers when
>>>they are not.
>>
>>Pointer taken!
>>
>>So you want to reserve the word "pointer" specifically to where a
>>memory address is involved on a Von Neumann architecture?

>
>
> That's a little narrow. I think the issue you mentioned
> earlier, the reference/dereference operator, is more
> important. Note also that a pointer is something that
> is only useful in the context of a particular address
> space; without the address space the pointer might
> as well be a random number. Whereas a foreign
> key does not so readily lose its meaning.
>
>
>
>

>>Since I was only drawing an analogy, I don't particularly see the
>>merit in being careful with the distinction between "pointer" and
>>"reference".

>
>
> I think it's safe to make the analogy. But don't lose the fact
> that the analogy doesn't make them the same.
>
>
>

>>However, I don't think it's bad at all that you want to be precise
>>with terminology.
>>
>>I could formalise "the equivalent of pointers" by defining an
>>isomorphism between a C based pointer implementation of an AST, and an
>>RM representation using arbitrary meaningless node identifiers, and
>>where pointer dereferences in the C implementation map to
>>corresponding joins in the RM representation.

>
>
> I am skeptical that such an isomorphism exists.

I am not skeptical. Check out the weasel words "arbitrary meaningless node identifier". If we say that arbitrary meaningless node identifiers point at nodes, then we have the isomorphism.

The real question to ask is: Why the hell would we need arbitrary meaningless node identifiers in the first place? Received on Wed Oct 31 2007 - 09:55:44 CDT