A Flexible Natural Language Parser Based on a Two-Level Representation of Syntax

A FLEXIBLE NATURAL LANGUAGE PARSER BASED ON A TWO-LEVEL REPRESENTATION OF SYNTAX Leonardo Lesmo and Pietro Torasso Istituto di Scienze dell'Informazione Universit~ di Torino C.so Massimo D'Azeglio 42 - 10125 TORINO - ITALY ABSTRACT In this paper we present a parser which al lows to make explicit the interconnections between syntax and semantics, to analyze the sentences in a quasi-deterministic fashion and, in many cases, to identify the roles of the various constituents even if the sentance is ill-formed. The main fea ture of the approach on which the parser is based consists in a two-level representation of the sy_n tactic knowledge: a first set of rules emits h~ potheses about the constituents of the sentence and their functional role and another set of rules verifies whether a hypothesis satisfies the con straints about the well-formedness of sentences. However, the application of the second set of rules is delayed until the semantic knowledge con firms the acceptability of the hypothesis. If the semantics reject it, a new hypothesis is obtained by applying a simple and relatively unexpensive "natural" modification; a set of these modifica tions is predefined and only when none of them is applicable a real backup is performed: in most cases this situation corresponds to a case where people would normally garden path. INTRODUCTION The problem of performing an accurate synta~ tic analysis of Natural Language sentences is still challenging for A.I. people working in the field of N.L. interpretation (Charniak 81, Kaplan 82). The most relevant points which attracted at tention recently are: the need of a strong connection between synta~ tic processing and semantic interpretation in order to reduce the space of the alternative sy~ tactic analyses (Konolige 80, Sidner et al. 81, Milne 82) - the convenience of a quasi-deterministic synta~ tic analysis, in order to reduce the computation al overhead associated with a heavy use of back up (Marcus 80) - the convenience of an approach which tolerates also (partially) incorrect sentences, at least when it is possible to obtain a meaningful inter pretation (Weischedel & Black 80, Kwasny & Sond heimer 81, Hayes 81). The first two of these remarks guided the design and the implementation of a system devoted to the interpretation of N.L. (Italian) commands (Lesmo, Magnani & Torasso 81a and 81b). In that system, however, as in most N.L. interpreters, the anal~ sis of the input sentence is mainly syntax-driven; for this reason, justin case the input sentence respects the constraints imposed by the syntactic knowledge it can be interpreted. The problem of analyzing ill-formed sentences has received a great deal of attention recently. However, most studies (Weischedel & Black 80, Kwasny & Sondheimer 81) are based on standard syn_ tactic analyzers (A.T.N.) which have been further ly augmented in order to take into account sen fences lacking some required constituents (elli~ sis) or where some syntactic constraints are not respected (e.g. agreement in number between the subject and the verb). There are two problems with this approach; both of …

Content truncated to 500 words