Interpreting Syntactically Ill-Formed Sentences

INTERPRETING SYNTACTICALLY ILL-FORMED SENTENCES Leonardo LESMO and Pietro TORASSO Dipartimento di Informatica - Universita' di Torino Corso Massimo D'Azeglio 42 - 10125 Torino - ITALY ABSTRACT The paper discusses three different kinds of syntactic ill-formedness: ellipsis, conjunctions, and actual syntactic errors. It is shown how a new grammatical formalism, based on a two-level repr_e sentation of the syntactic knowledge is used to cope with Ill-formed sentences. The basic control struc ture of the parser is briefly sketched; the paper shows that it can be applied without any substan tial change both to correct and to ill-formed sen tences. This is achieved by introducing a mechanism for the hypothesization of syntactic structures, which is largely independent of the rules defining the well-formedness. On the contrary, the second level of syntactic knowledge embodies those rules and is used to validate the hypotheses emitted by the first level. Alternative hypotheses are obtain ed, when needed, by means of local reorganizations of the parse tree. Sentence fragments are handled by the same mechanism, but in this case the second level rules are used to detect the absence of one (or more) constituents. INTRODUCTION In the last years we have been involved in building a natural language (Italian) interface to ward a relational database. Even if this research required to consider issues relative to knowledge representation (Lesmo et al 83) and query optimiza tion (Lesmo et al, in press), our main concern was to devise efficient parsing techniques (Lesmo et al 81, Lesmo & Torasso 83). The term "efficient", when applied to language processing, can take a number of different meanings, ranging from pure processing speed to the ability to analyze fragments of text, to the flexibility that characterizes the behavior of the parser. We believe that all facets of efficiency are worth be ing pursued, but if the communication between the man and the machine has to occur in a really natu ral fashion, the robustness of the parser, i.e. its ability to cope with unforeseen inputs must receive the greatest attention. It is important to realize that "unforeseen" is assumed her to refer to the syntactic form of the input sentence: of course, also inputs that are unexpected from a semantic point of view should be handled properly, but, since usually the syntactic knowledge acts as a fil ter between the reception of the input and the sub sequent stages of the analysis, the first problem that must be faced is the following: how can the parser be prevented from rejecting sentences that are syntactically ill-formed, but could be interpr_e ted correctly if they are passed to the other comp2 nents of the system? Alternatively, the problem can be stated as: how to foresee every interpretable input? Marcus (1982) envisages the following alternatives: a) the use of special "un-grammatical" rules, which explicitly encode facts about non-standard usage b) the use of "meta-rules" to relax the constraints imposed by classes of rules of the grammar c) allowing flexible interaction between syntax and semantics, so …

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