# RE: Noah Mendelsohn Comments on July 26 Draft of TAG Versioning Finding

From: <noah_mendelsohn@us.ibm.com>
Date: Tue, 5 Sep 2006 09:39:48 -0400
To: "Marc de Graauw" <marc@marcdegraauw.com>
Cc: "'David Orchard'" <dorchard@bea.com>, www-tag@w3.org
Message-ID: <OFDA4BADAD.17E49036-ON852571E0.004A0C90-852571E0.004B101A@lotus.com>
```
Marc de Graaw writes:

> (Noah Mendelsohn wrote:)
>
> | Why my strong preference?  Given this formulation, the test
> | for membership
> | of a text is merely a set membership test.  With Dave's, you
> | need to test
> | set membership, then syntactic legality, then map, then test
> | semantics. It
> | has to be harder to reason about all that.
>
> I agree with the notion that the set of texts in the language is
important.
> However I do not see whether it makes so much difference whether one
> enumerates those or intensionally defines them. To me defining a
language X
> as:
>
> X = {2, 3, 5, 7}
> or
> X = {x is a positive integer AND x is smaller than 10 AND x is a prime}
> seems pretty much equivalent.
>
> I guess the second is what David means when he says:
> | I think the key part is that the set of texts may be
> | determined by the constraints.
>
> However, it is true that one should not start with a set of texts bigger
> than those in the language and then constrain this set, so:
>
> X = {1, 2, 3, 4, 5, 6, 7, 8, 9}
> constraint: texts must be prime numbers
> is wrong. Constraints don't come on top of the set of texts, but they
can
> form an (non-enumerative) formulation of the set.

Exactly.   That's exactly what I was trying to say.  My preference is that
the language IS the set of texts.  How you convey the rules for what's in
the set is a separate issue, and intensional formulations are indeed just
fine.  Still, it's the set of texts that is the language, not the
(intensional) specification of the set, IMO.

> This point is important, but contains an unmentioned distinction:
between
> syntactical and semantical compatibility.

I agree, and I think that point is made in my original comments [1] on
Dave's draft.  For example, I proposed there:

"Text T is "incompatible" if any of the information in I2 is wrong (I.e.
was not present in I1 or replaces a value in I1 with a different
one...this rules disallows additional information, because only the
information in I1 is what the sender thought they were conveying, so
anything else is at best correct accidently). There are also intermediate
notions of compatibility: e.g. it may be that all of the information in I2
is correct, but that I2 is a subset of I1. [Not sure whether we should
name some of these intermediate flavors, but if we do, they should be
defined precisely.]"

These comments were intended to signal that we do indeed need to deal with
semantic as well as syntactic compatibility, and to point out that we can
get some mileage out of relatively formal, set-oriented approaches to the
information content as well.

> So the forwards/backwards relation is symmetrical with respect to
> syntax, but assymmetrical with respect to semantics.

Perhaps.  As I noted above, I think there are many useful "intermediate"
notions of compatibility, and we should try to provide names for some of
the common ones.  You may be write that semantic expectations for partial
forwards and backward compatibility tend to be different, but I think it's
still useful to start with a notion of absolute compatibility in each
direction, I.e. one in which the same text has the same exact meaning per
both languages.

[1]
http://lists.w3.org/Archives/Public/www-tag/2006Aug/att-0111/versioning26July2006withNoahComments.html

--------------------------------------
Noah Mendelsohn
IBM Corporation
One Rogers Street
Cambridge, MA 02142
1-617-693-4036
--------------------------------------
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Received on Tuesday, 5 September 2006 13:40:06 UTC

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