A General Computational Method For Grammar Inversion

A reversible grammar is usually understood as a computational or linguistic system that can be used both for analysis ~nd generation of the language it defines. For example, a directive pars_gen (Sent,For~n) would assign, depending upon the binding status Of its arguments, the representation in (Toronto,chased (Fido,John )) to the sentence Fido chased John in To~onto, or it would produce one of the several possib!e paraphrases of this sentence given its represen~tion. Building such bi-directional systems has long been considered critical for various natural language processing tasks, especially in machine translation. This paper presents a general computational method for automated inversion of a unification-based p~ser for natural language into an efficient generator. It clarifies and expands the results of earlier work on reversible grammars by this author and the others. A more powerful version of the grammar inversion algorithm is developed with a special emphasis being placed on the proper treatment of recursive ~rules. The grammar inversion algorithm described here is at the core of the Japanese-English :machine translation project currently under development at NYU. R E V E R S I B L E G R A M M A R S A reversible grammar is usually understood as a computational or linguistic system that can be used both for analysis ~ d generation of the language it defines. For : example, a directive pars_gen (Sent,Form) would assign, depending upon the binding status of its arguments, the representation in (Toronto, chased (Fido,John)) to the sentence Fido chased John in Toronto, or it would produce one of the several possibly paraphrases of this sentence given its representation. In the last several years, there have been a growing amount of research activity in reversibi¢ grammars for natural language, particularly in condecfion with machine translation work, and in natural language generation. Development of reversible 'grammar systems is considered desirable for variet), of reasons that include their immediate use in both parsing and generation, a reduction in the development and maintenance effort, soundness and completeness of linguistic coverage, as well as the match between their analysis and synthesis capabilities. These properties are important in any linguistic system, especially in machine translation, and in various interactive natural language systems where the direction of communication frequently changes. In this paper we are primarily interested in the computational aspects I of reversibility that include bi-directional evaluation and dual compilation of computer grammars, inversion of parsers into efficient generators, and derivation of "generating-versions" of existing parsing algorithms. Some of the recent resea~h in this area is reported in (Calder et al., 1989; Dymetman and Isabelle, 1988; Dymetman et al., 1990; Estival, 1990; Hasida and Isizaki, 1987; Ishizaki, 1990; Shieber, 1988; Shieber et al., 1990; Strzalkowski, 1990a-c; Strzalkowski and Peng, 1990; van Noord, 1990; and Wedekind, 1988). Dymetman and Isabelle (1988) describe a top-down interpreter for definite clause grammars that statically reorders clause literals according to a hand-eoded specification, and further allows for dynamic selection of AND goals 2 during execution, using the technique known as the goal freezing (Colmerauer, 1982; Naish, 1986). Shieber et al. (1990) propose a mixed top-down/bottom-up interpretation, in which certain goals, namely those whose expansion is defined by the so-called "chain rules", 3 are not expanded during the top-down phase of the interpreter, but instead they are passed over until a nearest non-chain rule is reached. In the bottom-up phase the missing parts of the goal-expansion tree will be filled in by applying i For linguistic aspects of reversible grammars, see (Kay, 1984; Landsbergen, 1987; Neuman, 1990; Steedman, 1987). 2 Literals on the fight-hand side of a clause create AND goals; literals with the same predicate names on the left-hand sides of different clauses create OR goals. 3 A chain rule is one where the main binding.carrying argument (the "head") is passed unchanged from the left-hand side to the fight. For example, assert(P) --> subj(Pl),verb(P2),obj(PI,P2,P), is a chain rule with respect to the argument P. assuming that P is the 'head' argument.