The assignment of grammatical relations in natural language processing

Abstract

THE ASSIGNMENT OF GRAMMATICAL RELATIONS IN NATURAL LANGUAGE PROCESSING Leonardo Lesmo, Vincenzo Lombardo Dipartimento di Infom~atica - Universila' di Tofino C.~ Svizzera 185 - 10149 Torino - ITALY e-mail: lesmo,vincenzo@di.unito.it 1. Introduction One of the main goals of an interpreter is to map the syntactic descriptions found in the sentence into the correct roles that the elements (described by the nominals) play in the situation at hand (described by the verb). For instance, we must be able to state that in 1) The cat ate the mouse the cat is the eater and the mouse is the eaten thing. Of course, if we only talk about roles and situations we miss some significant generalizations. In 2) The boy drank the water, if we say that the boy is the drinker and the water is the drunk thing, we disregard the evident similarity of the roles of eater and drinker in the two situations. The notion of deep case arises as the common ground underlying a number of apparently different roles. Upon this notion some frameworks, that stand at the core of semantic representation and natural language processing, are built (see [Fillmore 68], [Bruce 75] and ISomers 871). The hard task is to devise a mapping between the surface descriptions and these deep cases. The complexity of some syntactic phenomena, like passivization, and raising, long distance dependencies, has led many researchers to pose an intermediate level between the linear string of words and the case system. The concept involved is that of relation, such as subject, object, indirect object. It is claimed, for example, that passivizatiou is universally (cross-linguistically) explained if one says that the object of an active sentence becomes the subject in the passive form, rather than by saying that the NP in the VP is moved to replace the NP in S (that is a direct mapping). In the latter case it is implicit that the partictdar language under examination has a Subject- Verb-Object structure (SVO), as it usually happens in configurational languages such as English. In the example 3a) Lo hanno visto gli amici di Piero (Him &tve seen the friends of Piero) 3b) E' stato visto dagli amici di Piero ((He) has been seen by l'iero's friends) the passive form does not obey tile law of direct mapping. The example is, however, easily accounted fbr by the relational theories. The passivization rule induces only changes of function: the SUBJ becomes the BY- complement and the OBJ becomes the SUBJ. The importance of grammatical relations, taken as primitives for a universal grammar, is stated by a number of formalisms often collected under the label of Relational Grammar. The problem is to map the surface constituents into their correct roles. With languages as Italian, which stands in the middle between configurational and freely ordered languages [Stock 891 some flexibility is required to accomplish this task. One possibility is to adopt 11 neutral syntactic structure, open to several alternatives in the interpretation process. The head & modifier approach seems to feature this kind of neutrality, and has effectively been used for dealing with free word order languages, like the Slavonic languages [Sgall et al. 861 and Finnish [Jappinen et al. 86]. The dependency formalism we have adopted is presented in [Lesmo, Lombardo 91]. An example is reported in fig.l, and concerns the sentence: 4) La ragazza ebe lavora al guardaroba fu persuasa da un cliente a comprare una enciclopedia (The girl who works at the wardrobe was persuaded by a customer to buy an encyclopedia). The daughter nodes that stand on the left of their head precede it in the linear order of the sentence, while daughter nodes on the right follow it. The arcs that link the nodes in the dependency tree are of three types: arcs of structural and dependency (D&S arcs, represented by bold arrows in the figure), arcs of only structural dependency (STR arcs, simple arrows in the figure), and arcs of only dependency (DEP arcs, dashed arrows in the figure). D&S arcs link two words that stand in a both structural and logical relation. STR and DEP split these two functions of arc: an STR individuatcs a purely superficial AcrEs DE COLING-92. NAN'~S, 23-28 ^o~r 1992 1 0 9 0 PROC. or: COLING-92, NANTF.S. AUG. 23-28, 1992