Functional constraints in knowledge-based natural language understanding

Many knowledge-based systems of semantic interpretation rely explicitly or implicitly on an assumption of structural isomorphy between syntaotic and semantic objects, handling exceptions by ad hoc measures. In this paper I argue that constraint equations of the kind used in the LFG-(or PATR-)formalisms provide a more general, and yet restricted formalism :in which not only isomorphic correspondences are expressible, but also many cases of non-isomorphic correspondences. I illustrate with treatments of idioms, speech act interpretation and discourse pragmatics. 1. Background and purpose In knowledge-based natural language understanding systems the role of syntax is by no means self-evident. In the Yalean tradition /,~tchank & Riesbeck, 1981/ syntax has only played a minor role and whatever little syntactic information there is has been expressed in simple terms. Consequently, there is no grammar as such and syntactic conditions are freely intermixed with semantic conditions in the requests that drive the system forward/13irnbaum & Selfridge, 1981/. Similarly, in frame-based systems such as /Hayes, 1984/ the syntactic information is stated in ~:onjunction with all other information relevant for instances ot a frame. A justification for this approach, apart from transparency, is that it makes sense to say that part of our knowledge of a concept is knowledge about how it is communicated. A major disadvantage of this approach is of course its lack of generality. To overcome this problem we may extract general syntactic knowledge' and make use of it in a syntactic parser which works alongside with the semantic analyser. Examples of such systems are PSI-KLONE /Bobrow & Webber, 1980; Sondheimer et ah, 1984/ and MOPTRANS /Lytinen, 1986; 1987/. The promise of these systems is that you get both modularity and integration, although there are many open questions about how the integration can best be achieved. Moreover, one would I!ke to put the integration of syntax and semantics, not just syntax and semantics per se, on a principled basis, i.e. we need a theory of how syntactic and semantic objects correspond. Linguistics and philosophy offer some guidelines here, such as compoeitionaiity, and a number of different theories, but a problem is that the semantic objects considered are usually not knowledge structures. /Hirst, 1987/, though, is an attempt at a principled, modular and integrated knowledge-based system where compositionality and a principle of strong typing provide the theoretical underpinnings. These principles teem to provide a tighter straight-jacket than one would really want, however, as indicated by …