An Experimental Parser for Systemic Grammars

An Experimental Parser for Systemic Grammars Robert T. KASPER USC/information Sciences Institute 4676 Admiralty Way, Suite 1001 Marina del Rey, CA 90292 U.S.A. Abstract We descrlbe a general parsing method for systemic gram- mars. Systemic grammars contain a paradigmatic analysis of language in addition to structural information, so a parser must assign a set of grammatical features and functions to each constituent in addition to producing a constituent struc- ture. Our method constructs a parser by compiling systemic grammars into the notation of Functional Unification Gram- mar. The existing methods for parsing with unification gram- mars hace been extended to handle a fuller range of paradig- matic descriptions. In particular, the PATR-II system has been extended by using disjunctive and conditional information in functional descriptions that are attached to phrase structure rules. The method has been tested with a large grammar of En- glish w:hich was originally developed for text generation. This testing is the basis for some observations about the bidirec- tional m~e of a grammar. 1 Introduction Many computational linguists have found systemic grammar (SG) to be quite useful, because it provides an explicit representation of features that determine how a sentence functions in the context of communication. SG has been used directly as the basis for several computer tex~ generation programs [Mann 82], but it has only been used indirectly for computational parsers. Winograd used principles from SG in designing SHRDLU [Winograd 72], a successful natural language understanding program, but his program did not contain an explicit representation of the grammar's system network. Instead, he used a special purpose programming language to encode gram- matical knowledge in a procedural form tailored specifically to the language understanding task. Another procedural implementation of SG was developed by McCord [McCord 77]. Both of these methods would require a significant progranmfing step before a parser could be produced for a different grammar. Our goal has been to develop a general parsing method using a declarative representation of SG, and to determine to what extent a grammar that is adequate for text generation can also be used for text analysis. Our parser has been developed and tested using Nigel [Mann 83], a large grammar of English that has previously been used as part of a text generation system. Systemic l;nguistics builds on the foundation of Hailiday's con.. cept of the system Setwork [Halliday 76]. A systemic grammar is organized around choices between grammatical features that reflect the structure and content of aconstituent. Each choice between fea- tures is called a system. Thus, a systemic grammar has two major components: I. a system network of feature choices, and 2. structurM realization statements corresponding to each feature. The feature Choices define the options available to be expressed in RANK -Clause ~-... MOOD TYPE a language, and may be regarded as "hooks" into a semantic component. The realization statements determine the constituent structure. There are realization statements to declare the presence of constituents, conflate constituents, specify feature constraints on constituents, and specify ordering constraints among …

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