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Re: RM and abstract syntax trees

From: David BL <davidbl_at_iinet.net.au>
Date: Wed, 31 Oct 2007 05:31:28 -0700
Message-ID: <1193833888.092224.27420@k35g2000prh.googlegroups.com>


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? Since it's merely a terminology issue I won't disagree with you. I presume you would say that a C++ smart pointer that binds to its referenced object using a key in a red black tree must not under any circumstances be regarded as somehow being the "equivalent" of a pointer or being "analogous" to a pointer.

The following is quoted from http://en.wikipedia.org/wiki/Pointer

    Pointers are so commonly used as references that sometimes     people use the word "pointer" to refer to references in general

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

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.

Note BTW that mathematicians may even go beyond the term "equivalent" and use "same" when they see an isomorphism. For example the axioms of the reals only make them unique up to isomorphism, yet we say *the* reals. Received on Wed Oct 31 2007 - 07:31:28 CDT

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