Default unification: online vs. offline

[Stephen M. Wechsler]

This message from Mark Johnson, responding to my question about the
feature ANY, may have bounced from the list:

At 12:20 PM -0500 11/16/01, Mark Johnson wrote:
>Hi Steve,
>
>I don't know whether HPSG still contains ANY features (either directly or
>indirectly), but you're right that it causes a certain amount of
>theoretical grief.  The easiest way of understanding ANY features involves
>the notion of two different evaluation ``times'' (you solve the
>constraints ignoring the ANYs, then check to see if the minimal model
>solutions satisfy the constraints with the ANY features), which is
>horribly non-declarative.
>
>I wrote a number of papers describing how to express ANY constraints in
>various kinds of default logics.  To the extent to which we can give a
>declarative semantics to these default logics, we have a declarative
>semantics for ANY constraints.  Unfortunately, the semantics of
>constraint logics is itself not exactly a picture of clarity:  As Samuel
>Johnson (I think it was him) said about the dog walking on its hind legs,
>it's amazing that it can be done at all, but it's still not what I'd call
>a success.
>
>However, I think it's also worth asking what the implications are of not
>having a nice simple declarative semantics.  While having a simple
>declarative semantics is undoubtedly better than not having one, having a
>simple declarative semantics has little to do with the insightfulness of
>linguistic analyses you (can) use the theory to express, or the
>computational complexity of parsing or generating using that theory.
>
>In other words, I guess I now believe that if having ANY values or
>equivalent devices permits simpler, more insightful linguistic analyses,
>then by all means use them, and let the mathematicans worry about the best
>way to model them formally,
>
>Best
>
>Mark

This is what I wanted to hear.

Our of curiosity, then, why is this mechanism not considered a problem for LFG?

Steve


>On Thu, 1 Nov 2001, Stephen M. Wechsler wrote:
>
>>  While we're on this general topic, I wonder if someone could tell me
>>  why the feature 'ANY' has disappeared from HPSG, or whether it still
>>  exists in some other guise.  The idea, if I remember right, is
>>  essentially like an LFG constraining equation.  ANY must unify with
>>  something, or the representation is ill-formed.  So it is sort of an
>>  output filter.  Is there a profound theoretical problem with this
>>  idea?  If so, then why doesn't LFG have the same problem?  This
>>  device is very useful for certain applications.
>>
>>  Steve
>>
>>  --
>>
>>  Stephen Wechsler, Assoc.Prof. |   http://uts.cc.utexas.edu/~wechsler/
>>  Linguistics Dept.             |   Calhoun Hall 403
>>  University of Texas           |   direct ph. (512) 232-7683
>>  Austin, TX 78712-1196         |   wechsler at mail.utexas.edu
>>  dept. ph. (512) 471-1701      |   Fall 2001 Office hrs:
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>  >