Formal Specification of Natural Language Syntax Using Two-Level Grammar

FORMAL SPECIFICATION OF NATURAL LANGUAGE SYNTAX ABSTRACT The two-level grammar is investigated as a notation for giving formal specification of the context-frec and context-sensitive aspects of n,~tural language syntax. In this paper, a large class of English declarative sentences, including post-noun-modificatlon by relative clauses, is formalized using a two-level grammar. The principal advantages of two- level grammar are: 1) it is very e~sy to understand and may be used to give a formal description using a structured form of natural language; 2) it is formal with many well-known mathematical properties; and 3) it is directly implementable by interpretation. The significance of the latter fact is that once we have written a two-level grammar for natural language syntax, we can derive a parser automatically without writing any additional specialized computer programs. Because of the ease with which two-levcl grammars may express logic and their Turing computability we expect that they will also bc very snitable for future extensions to semantics and knowledge representation. 1. INTRODUCTION Formal specifications of natural language syntax should serve as a standard definition for the syntax of the subject language. The specification must be complete, concise, consistent, precise, unambiguous, understandable, and useful to language scholars, users, and implcmentors who wish to develop a parser for the tanguagc to run on a computer. Furthermore the specification should be mathematically rigorous to the degree that an implementation of the language can be automatically derived from the specification {10]. Unfortunately many of these aims arc difficult to accomplish primarily because of the dynanric and informal nature of natural language. Formal specification is still a worthy goal to the degree allowed by present knowledge about natm'al language and iu this paper we propose a mctalanguage for specifying both syntax and semantics of natural language that has potential for satisfying these goals. The mctalanguage we propose is the two-levd grammar [16} (also called W-grammars and tlgs). Two-level grammars have been used extensively for specifying the syntax and semantics of programming languages [2] but their use in specifying natural language was first introduced by the authors [7, 8, 9]. Existing formal specification mcthods for natural language syntax take many forms. Of these, some of the more common are augmented transition network grammars [181, transformational grammars [1], and generalized phrase-structure grammars [5]. These methods and others arc also surveyed in [17]. The degree to which any formal specification method satisfies the above stated goals is sometimes difficult to evaluate and relies on subjectivity. The authors do not intend to evaluate these existing methods with respect to the requirements of formal specification languages but will instead concentrate on why two-level grammars satisfy the necessary goals in a mathematically rigorous but readable and easy to understand way. In this paper, the two-level grammar mctalanguage will be used to define a large classification of English declarative sentences, extending work described in [8] and [9]. We will emphasize the method of using two-level grammars for this purpose and the advantages gained rather than any particular characteristics of the …

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