A prolog implementation of lexical functional grammar as a base for a natural language processing system

Abstract

The aim of this paper is to present parts of our system [2], which is to construct a database out of a narrative natural language text. We think the parts are of interest in their own. The paper consists of three sections:(1) We give a detailed description of the PROLOG - implementation of the parser which is based on the theory of lexical functional grammar (LFG). The parser covers the fragment described in [1,§;4]. I.e., it is able to analyse constructions involving functional control and long distance dependencies. We will to show that- PROLOG provides an efficient tool for LFG-implementation: a phrase structure rule annotated with functional schemata likes S → NP VP is to be interpreted as, first, indetifying the special grammatical relation of subject position of any sentence analyzed by this clause to be the NP appearing in it, and second, as identifying all grammatical relations of the sentence with those of the VP. This universal interpretation of the LFG-metavariables ↑ and ↓ corresponds to the universal quantification of variables appearing in PROLOG-clauses. The procedural semantics of PROLOG is such that the instantiation of the variables in a clause is inherited from the instantiation given by its subgoals, if they succeed. Thus there is no need for a separate component which solves the set of equations obtained by applying the LFG algorithm.-there is a canonical way of translating LFG into a PROLOG programme.(II) For the semantic representation of texts we use the Discourse Representation Theory developped by Hans Kamp. At present the implementation includes the fragment described in [4]. In addition it analyses different types of negation and certain equi- and raising-verbs. We postulate some requirements a semantic representation has to fulfill in order to be able to analyse whole texts. We show how Kamp's theory meets these requirements by analyzing sample discourses involving anaphoric NP's.(III) Finally we sketch how the parser formalism can be augmented to yield as output discourse representation structures. To do this we introduce the new notion of 'logical head' in addition to the LFG notion of 'grammatical head'. The reason is the wellknown fact that the logical structure of a sentence is induced by the determiners and not by the verb which on the other hand determiners the thematic structure of the sentence. However the verb is able to restrict quantifier scope ambiguities or to induce a preference ordering on the set of possible quantifier scope relations. Therefore there must be an interaction between the grammatical head and the logical head of a phrase.