Coping with Extragrammaticality

Coping with Extragrarnmaticality Jalme G. Carbonell and Philip J. Hayes Computer Science Department, Carnegie-Mellon University Pittsburgh, PA 15213. USA Abstract 1 Practical natural language interfaces must exhibit robust bei~aviour in the presence of extragrammaticat user input. This paper classifies different types of grammatical deviations and related phenomena at the lexical and sentential levels, discussing recovery strategies tailored to specific phenomena in the classification. Such strategies constitute a tool chest of computationally tractable methods for coping with extragrammaticality in restricted domain natural language. Some of the strategies have been tested and proven viable in existing parsers. 1. Introduction Any robust natural language interface must be capable of processing input utterances that deviate from its grammatical and semantic expectations. Many researchers have made this observation and have taken initial steps towards coverage of certain classes of extragrammatical constructions. Since robust parsers must deal primarily with input that does meet their expectations, the various efforts at coping with extragrammaticality have generally been structured as extensions to existing parsing methods. Probably the most popular approach has been to extend syntactically.oriented parsing techniques employing Augmented Transition Networks (ATNs) [21, 24, 25, 29]. Other researchers have attempted to deal with ungrammatical input through network-based semantic grammar techniques [19. 20j. through extensions to pattern matching parsing in which partial pattern matching is allowed [16], through conceptual case frame instantiafion [12, 22], and through approaches involving multiple cooperating parsing strategies [7, 9, 18]. Given the background of existing work, this paper focuses on two major objectives: 1. to create a taxonomy of grammatical deviations covering a broad range of extragrammaticalities, 2. to outline strategies for processing many of these deviations, 3. to assess how easily these strategies can be employed in conjunction with existing parsing methods. The overall result should be a synthesis of different parse. recovery strategies organized by the grammatical phenomena they address (or violate), an evaluation of how well the strategies • integrate with existing approaches to parsing extragrammatical 1This research was sponsored in part by the Air Force Office of Scientific Research under Contract AFOSR-82-0219 and in part by Digital Equipment Corporation as part of the XCALIBUR project. input, and a set of characteristics desirable in any parsing process dealing with extragrammatical input. We hope this will aid researchers designing robust natural language interfaces in two ways: t.by providing a tool chest of computationally effective approaches to cope with extragrammaticality; 2. by assisting in the selection of a basic parsing methodology in which to embed these recovery techniques. In assessing the degree of compatibility between recovery techniques and various approaches to parsing, we will avoid the issue of whether a given recovery technique can be used with a specific approach to parsing. The answer to such a question is almost always affirmative. Instead, we will be concerned with how naturally the recovery strategies fit with the various parsing approaches. In particular, we will consider the computational tractability of the recovery strategies and how easily they can obtain the information they need to operate in …

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