An English Generator for a Case-Labelled Dependency Representation

.~N F~GLISH GF~NERATOR FOR A CASE-LABELLED DEP~qD~QCY REPRESENIW£10N John Irving Tait Acorn Computers Ltd. Fulboum~ Road Cherry Hinton Cambridge CB1 4JN U.K. Abstract The paper describes a progrmn which has been constructed to produce English strings from a case-labellea depenaency representation. The program uses an especially single and uniform control structure with a well defined separation of the different knowledge sources used during ge~,eration. Furthermore, the majority of t/le syst~n's knowledge is expressed in a declarative form, so in priciple the generator ' s knowledge bases could be used for purposes other than gex,eration. The ge~erator uses a two-pass control structure, the first translating from the s~nantically orientated case-labelled dependency structures into surface syntactic trees and the second translating from these trees into English str i~/s. The generator is very flexible: it can be run in such a way as to produce all the possible syntactically legitimate variations on a giveJ, utterance, and has built in facilities to do some synon~s substitution. It has been used in a nu, ber of application domains: notably as a part of a free text retrieval system and as part of a natural language front end to a relational database system. i. Introduction This pa[~er describes a progrmn which has been constructed to translate from Boguraev ' s case-labelled depe~idency representations (Boguraev, 1979: see also 8oguraev and Sparck Jones, 1982) to English strings. Although the principles on which the program has been constructed are primarily a new mix of established idea~, the generator incorporates a number of novel features. In particular, it caLlbines an especially simple a,~ uniform control structure with a well defined separatlon of t/le differe~,t ka,owledge sources used du~ing generation. It operates in two passes, the f~rst translating from the semantically orientated case-labelled dependency structures into surface syntactic trees a~,d the secona translating fran these trees into English strings. The translation fran de[~_ndency structures to surface syntactic trees is the more c~mplex of the two passes unaertaken by the generator a~ will be described here. The other, translation from instantiated surface trees to text strings is relatively straightforward and will not be dealt with in this paper. It is fundamentally a tree flattening process, and is described in detail in Tait and Sparck Jones (1983). 2. The Generator's Knowledge Structures The generator's Knowledge is separated into four sections, as follows. i) a set of bare templates of phrasal and clausal structures which restrict the surface trees other parts of the system may produce by defining the branching factor at a giv~_n node type. For example, the patterns record that English has intransitive, transitive and ditransitive, but not tritraneitive, verb phrases. The bare template for noun phrases is illustrated in Figure i. 2 ) a lexicon and an associated morphological process~. 3) a set of productzon rules which fill out partially instantiated syntactic trees produced from the phrasal ~,~ clausal patterns. These rules contain most of the syst~n's knowleuge about the relatzonship between the constructs of Boguraev' s representation la,~uage and English …

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