[developers] Questions about the MRS algebra from Seattle

[Ann Copestake]

Hi Emily,

Also quick replies ....

On 29/06/2015 17:46, Emily M. Bender wrote:
> Dear Ann,
>
> Thanks for the quick answers!  Some further comments/questions below:
>
> On Fri, Jun 26, 2015 at 4:09 PM, Ann Copestake <aac10 at cl.cam.ac.uk 
> <mailto:aac10 at cl.cam.ac.uk>> wrote:
>
>     here's some quick answers (on the basis I may never get round to
>     replying if I try and reply more carefully)
>
>     On 26/06/2015 22:13, Emily M. Bender wrote:
>
>         Dear all,
>
>         The UW group has been reading and discussing Copestake et al 2001
>         and Copestake 2007, trying to get a better understanding of
>         the MRS
>         algebra.  We have a few questions---I think some of these
>         issues have been
>         proposed for the Summit, but I'm impatient, so I thought I'd
>         try to get
>         a discussion going over email.  UW folks: Please feel free to
>         chime in
>         with other questions I haven't remembered just now.
>
>         The two big ones are:
>
>         (1) Copestake et al 2001 don't explicitly state what the
>         purpose of the
>         algebra is.  My understanding is that it provides a guarantee
>         that the MRSs
>         produced by a grammar are well-formed, so long as the grammar is
>         algebra-compliant.   Well-formed MRS (in this sense) would
>         necessarily
>         have an interpretation because the algebra shows how to
>         compose the
>         interpretation for each sement.  Is this on track? Are there
>         other reasons
>         to want an algebra?
>
>
>     We have never managed to prove that MRSs constructed according to
>     the algebra will be scopable, but I think that is the case.  But
>     more generally, the algebra gives some more constraints to the
>     idea of compositionality which isn't the case if you simply use
>     feature structures.  It excludes some possible ways of doing
>     semantic composition and therefore constitutes a testable
>     hypothesis about the nature of the syntax-semantics interface.  It
>     also allows one to do the same semantic composition with grammars
>     in formalisms other than typed feature structures.
>
>
> I'm still trying to understand why it's important (or maybe interesting is
> the goal, rather than important?) to exclude some possible ways of doing
> semantic composition.  Are there ways that are problematic for some 
> reason?
> Is it a question of constraining the space of possible grammars (e.g. for
> learnability concerns)?  Related to issues of incremental processing?

The following may sound snarky but I really don't mean it this way - 
would you have the same type of questions about syntactic formalism?  
Because I can answer in two ways - one is about why I think it's 
important to build testable formal models for language (models which 
aren't equivalent to general programming languages, so more constrained 
thn typed feature structures) and the other is about what the particular 
issues are for compositional semantics.  I don't want to reach the 
conclusion that semantic composition requires arbitrary programs without 
looking at more constrained alternatives.  So yes: learnability, 
processing efficiency (human and computational), incrementality and so 
on, but one can't really look at these in detail without first having an 
idea of plausible models.

>
>         Subquestions:
>
>          (1a) I was a bit surprised to see the positing of labels in
>         the model.  What
>         would a label correspond to in the world?  Is this akin to
>         reification of propositions?
>         Are we really talking about all the labels here, or just those
>         that survive once
>         an MRS is fully scoped?
>
>
>     the model here is not a model of the world - it's a model of
>     semantic structures (fully scoped MRSs).
>
>
> Oh, I missed that in the paper.  So are we then talking about 
> "interpretation"
> because the fully scoped MRSs are assumed to have interpretations via
> a second layer of modeling?

yes

>          (1b) How does this discussion relate to what Ann was talking
>         about at IWCS
>         regarding the logical fragment of the ERG and the rest of the
>         ERG?  That is,
>         if all of the ERG were algebra-compliant, does that mean that
>         all of the ERSs
>         it can produce are compositional in their interpretation? Or
>         does that require
>         a model that can "keep up"?
>
>
>     it's really orthogonal - what I was talking about at IWCS was
>     about the complete MRSs.
>
>
> Got it.
>
>         (2) Copestake et al state: "Since the constraints [=
>         constraints on grammar rules
>         that make them algebra-compliant] need not be checked at
>         runtime, it seems
>         better to regard them as metalevel conditions on the
>         description of the grammar,
>         which can anyway easily be checked by code which converts the
>         TFS into the
>         algebraic representation."  What is the current thinking on
>         this?  Is it in fact
>         possible convert TFS (here I assume that means lexical entries
>         & rules?) to
>         algebraic representation?  Has this been done?
>
>
>     `easily' might be an exaggeration, but the code is in the LKB,
>     though it has to be parameterised for the grammar and may not work
>     with the current ERG. You can access it via the menu on the trees,
>     if I remember correctly.  The small mrscomp grammar is algebra
>     compliant, the ERG wasn't entirely when I tested it.
>
>
> In the spirit of keeping the discussion going without delays, I 
> haven't actually
> played with this yet.  But: accessible from the trees seems to suggest 
> that the
> testing takes place over particular analyses of particular inputs, and 
> not directly
> on the grammar as static code analysis.  Is that right?
>

I think one could use static analysis based on that code on a grammar 
which was very careful about not having pointers into the semantics 
other than those licensed by the algebra.  Either no such pointers at 
all, or ones in which there was a clear locality, so it was never 
possible for information to sneak back into the semantics bypassing the 
official composition.   Proving that can't happen with a grammar like 
the ERG is difficult.

> Thanks again,
> Emily
>

you're welcome!

All best,

Ann

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