Modular Logic Grammars

MODULAR LOGIC GRAMMARS Michael C. McCord IBM Thomas J. Watson Research Center P. O. Box 218 Yorktown Heights, NY 10598 ABSTRACT This report describes a logic grammar formalism, Modular Logic Grammars, exhibiting a high degree of modularity between syntax and semantics. There is a syntax rule compiler (compiling into Prolog) which takes care of the building of analysis structures and the interface to a clearly separated semantic interpretation component dealing with scoping and the construction of logical forms. The whole system can work in either a one-pass mode or a two-pass mode. [n the one-pass mode, logical forms are built directly during parsing through interleaved calls to semantics, added automatically by the rule compiler. [n the two-pass mode, syn- tactic analysis trees are built automatically in the first pass, and then given to the (one-pass) semantic component. The grammar formalism includes two devices which cause the automatically built syntactic structures to differ from derivation trees in two ways: [I) There is a shift operator, for dealing with left-embedding constructions such as English possessive noun phrases while using right- rezursive rules (which are appropriate for Prolog parsing). (2) There is a distinction in the syn- tactic formalism between strong non-terminals and weak non-terminals, which is important for distin- guishing major levels of grammar. I. INTRODUCTION l'he term logic grammar will be used here, in the context of natural language processing, to mean a logic programming system (implemented normally in P£olog), which associates semantic represent- ations Cnormally in some version of preaicate logic) with natural language text. Logic grammars may have varying degrees on modularity in their treatments of syntax and semantics. Th, ere may or may not be an isolatable syntactic component. In writing metamorpilosis grammars (Colmerauer, 1978), or definite clause grammars, DCG's, (a spe- cial case of metamorphosis grammars, Pereira and Warren. 1980), it is possible to build logical forms directly in the syntax rules by letting non- terminals have arguments that represent partial logical forms being manipulated. Some of the ear- ties= logic grammars (e.g., Dahl, 1977) used this approach. There is certainly an appeal in being dicect, but there are some disadvantages in this lack of modularity. One disadvantage is that it seems difficulZ to get an adequate treatment of the scoping of quantifiers (and more generally focalizers, McCord, 1981) when the building of log- ical forms is too closely bonded to syntax. Another disadvantage is just a general result of lack of modularity: it can be harder to develop and un- derstand syntax rules when too much is going on in them. The logic grammars described in McCord (1982, 1981) were three-pass systems, where one of the main points of the modularity was a good treatment of scoping. The first pass was the syntactic compo- nent, written as a definite clause grammar, where syntactic structures were explicitly built up in the arguments of the non-terminals. Word sense selection and slot-filling were done in this first pass, so that the output analysis trees were actu- ally partially semantic. …

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