Parsing with on-line principles: a psychologically plusible, object-oriented approach

Paral le l ing recen t shifts within Grammatical Theory away from rule-based and toward principle-based systems, there has arisen widespread interest in the possibility of similar refocusing with respect to natural language processing (cf. Abney (1988), Berwick & Weinberg (1984), Clark (1987), Fong (1990), Gibson (1987), Johnson (1988), Kashket (1988), Pri tchett (1987, 1988, 1990, in press , forthcoming), Stabler (1989), among others). Fundamental to principle-based as opposed to rule-based models of parsing is the hypothesis that the Parser itself adheres to a version of the Projection Principle which maintains that each level of syntactic representation is a projection of the lexical properties of heads. With respect to parsing, the PP implies that a node emmet be projected before the occurrence of its head since the relevant features which determine its categorical identity and license its own and its a rguments ' a t tachment are theretofore undetermined. This paper describes an ongoing project in the implementation of an object-oriented (Smalltalk-80 TM) Government and Binding parser which adheres to Ihe strong competence hypothes i s that principles of Universal Grammar are emp!oyed directly in parsing. Specifically, the parse operates by projecting phrasal structure as determined by the lexical properties of heads and licensing local attachments which maximally satisfy on-line principles of Universal Grammar at every point during a parse. Though this model was originally mot iva ted with regard to its psychological plausibility, in this paper we tbcus primarily on issues of implementation (see Pritchett: op. cir. for a more detailed discussion of the psycho!inguistic issues). In the implemented parser, the following new O b j e c t subclasses are defined: Object PrincipleBasedParser Lexicon LexicalItem Node EmptyNode FullNode DoubleBarNode SingleBarNode ZeroBarNode Chain LicensingRelation ThetaRoleAs signment CaseAssignment SpecHe adAgreement XPSe lection An instance of PrincipleBasedParser (henceforth simply the parse,') itself7 acts as the buffer for tree structures. The parse of a string succeeds if at the end of input, there is exactly one tree in the parser and all grammatical principles are satisfied for every Node in that tree. The syntactic structures actually created and manipulated by the parser are subinstances of the class Node. Nodes accord with a binarybranching version of X' Theory and each Node exists as an element of a maximal projection: [xP [vP ] [x' [X] [zP ]]]. Phrase Structure constraints on the linear order of Nodes is specified in the pool variable, HeadParameter; in this note we assume the English configuration. The spec i f i e r and complement positions themselves are either fully specified maximal projections or instances of the special class EmptyNode. Nodes respond in the expected fashion to a range of messages concerning eonf igu ra t iona l s t ruc ture , such as c commands :, m-commands ::, governs:, mother, sister, etc. Each Node may be associated (coindexed) wittl other Nodes via an instance of the class c h a i n , a subclass of S o r t e d C o l l e c t i o n , where Node ct precedes Node 13 in an instance