The IPS System

The IPS system is a large-scale interactive GB-based parsing system (English, French) under development at the University of Geneva. This paper starts with an overview of the system, discussing some of its basic features as well as its general architecture. We then turn to a more detailed discussion of the "right corner" parsing strategy developed for this project. Combining top down and bot tom up features, this s trategy is consistent with an incremental interpretat ion of sentences. 1 O v e r v i e w o f t h e I P S p r o j e c t The IPS (Interactive Parsing System) research project, at the Linguistics Departement of the University of Geneva, aims at developing a large, interactive, parsing model based on Chomsky's Government and Binding (henceforth GB) linguistic theory t. This project, which focuses both on English and French, has theoretical as well as practical goals, including the following: • to show the feasibility and the soundness of a GB-based approach to natural language parsing; *The research described in this paper has been supported in part by n grant from the Swiss national science foundation (grant no 11-25362.88). I am grateful to Robin Clark, Paola Merlo~ Mira Ramluckun and Martin Kay for helpful comments and discussion on earlier drafts of this paper. C], Chonmky (1981, 1988) for a discussion of 6B theory. Issues related to the use of Gl] theory in natural language parsing delign are discussed in Berwick (1887), Berwick et ol. (1991) mad Wehdi (1988), • to demonstrate the advantages of an interactive approach to natural language processing; a to develop robust, large-scale parsers suitable for NLP applications (e.g. translation). The IPS parser is interactive in the sense that it can request on-line information from the user. Typically, interaction will be used to solve ambiguities that the parser cannot handie, for instance when the resolution of an ambiguity depends on contextual or extra-linguistic knowledge 2. The interactive feature is seen as a way to increase the reliability of the parser difllcult decisions are left to the user as well as a way to simplify the grammar, since many ad hoc features that would be necessary if the parser had to solve all problems by itself can now be dispensed with. In addition, the interactive capabillty is also useful as a development tool, in the sense that on-line user interaction can supplement modules which have not yet been developed (e.g. semantic and pragmatic components). Other important features of the IPS parser include: • Modular architecture, i.e. the parsing mechanism is decomposed into modules, which roughly correspond to some of the components of a s tandard GB grammar (e.g. X, chains, O, etc.). • The parsing strategy is left-to-right, data driven, with parallel t reatment of alterna2For arguments in favor of interactive systems (usually in the context of machine translation, though) see in particular Kay (11180), Melby et al. (1980), Tomita (1984), Wehrli (1990), Zajac (1988). Acres DE COLING-92, NANTES, 23-28 AOt3"r 1992 8 7 0 PROC. OF COL1NG-92, NANTES, AUO. 23-28, 1992 fives. The nonodeterminism of the parser is restricted by a selection mechaafism. a Use of structure-sharing techniques to cut down the number of explicit representations of alternatives. 2 A r c h i t e c t u r e The IPS parser tries to associate with an input sentence a set of syntactic structures. These structures correspond to GB S-structures, i.e. surface structures enriched with traces of moved elements and other empty categories. In our implementation, GB grammatical modules correspond to particular processes. While some of the modules function as generators (2, chain modules, coordination module) in the sense that they increase the set of structures hypothesized at a given point, others are used as filters (Case module, 0-module) in the sense that their action tends to reduce the set of structures. The modules apply as soon as possible at each step in the parsing process, triggered by particular data or by specific calls from other modules. Alternatives are considered concurrently (pseudo-parallelism), and a small number of heuristics are used to restrict the size of the hypothesis set. To give an example, one heuristic gives precedence to attachments satisfying forreal selectional features over (cf. (3), (7)) other kinds of attachments. Thus, if an incoming verb form can be attached either as a complement to an auxiliary or as a main verb, preference will be given to the former a. User interaction -wtfich is an optional feature in the IPS systemcan be used to select alternatives, mostly in case of attachment ambiguities, but occasionally also for other types of ambiguity (lexical, thematic, ere). Alternatives are then displayed (in an abbreviated manner) and the user is asked to make a selection. aNotice that this heuristic might explain garden path sentences such as "l'invitd quail a dit des folies" (the gueJt he has told inzanitiez),in which readers wrongly interpret dit as past participle selected by the auxiliary verb a. 3 T h e X m o d u l e The central module of the IPS system is the module, which acts as the main generator of the system, and determines the syntactic structures of constituents. We assume the X schema in (1): (1) XP --~ Spec X-+ X Compl where X is a lexical or a functional category, Spee and Compl are lists (possibly empty) of maximal projections (YP). As indicated in (1), maximal projections are of order 2 (XP) and lexicai categories of order 0 (I). For typographical reasons, categories of order 1 (~) are noted •' in the illustrations below. The set of lexical categories include N, V, • and p, the set of functional categories includes D(eterminer), T(ense) et C(omplementizer). We also assume the DP hypothesis (cf. Abney 1987, Clark 1990a), and, as a consequence, the strong parallelism between DP and TP structures, as illustrated in (2):