Syntactic Choice in Language Generation

SYNTACTIC CHOICE IN LANGUAGE GENERATION r Lee Fedder, University of Cambridge, Computer Laboratory, Pembroke Street, Cambridge CB2 3QG, England. lf~uk.ac.cam.cl 1 Abstract A considerable amount of study has recently been concentrated on the use of linguistically motivated unification based grammars for sen- tence generation (see for example Appelt 1987, Calder 1989, Shieber 1988, Shieber et al. 1989). The grammars state correspondences between semantic structures and syntactic ones. In most grammars, several syntactic structures will correspond to each semantic one. It was suggested at a fairly early stage that control over which sentence is generated could be ap- plied by adding "functional" features to the grammar. This idea has been extended in the work presented here. The use of feature threading techniques allows control over a wide range of syntactic structures, in a fairly sophisti- cated grammar, while avoiding the need for rule duplication even when generating un- bounded dependencies. However, we will see that the feature system required quickly be- comes complex, and: may be difficult to ex- tend to more comprehensive grammars. 2 Introduction "A thematic system is one where corresponding members of the con- trasting terms normally have the same propositional meaning, and the same illocufionary potential." (Huddleston 1984:p437 ). Most phrase structure or categorial unifi- cation based grammars encode some form of thematic system. The simplest would involve the inclusion of both active and passive voice. Typically, the grammar defines the syntactic structure of each form, but does not include the pragmatic information that distinguishes one from another. When using such a gram- mar for parsing, this is not important, so long as the information is not required by the system using the parser's output. However, there has recently been an upsurge in the use of these grammars for generation. The lack of pragmatic data now becomes important: the generator is under-constrained, being ca- pable of producing any of the available the- matic forms. One way of applying the neces- sary constraints, is to introduce a system of "functional" features into the feature struc- ture of the grammar itself. These features are so called because they refer to the function of the various parts of the sentence in a dis- course. McKeown suggested the use of func- tional features for the TEXT system (McKe- own 1985) in which the grammar was based on the FUG formalism (Kay 1979). The func- tional features were defined as part of the ini- tial specification of the sentence, which was then filled out by traversing the grammar in a "Top Down" fashion. For example, the fol- lowing was given by McKeown as an initial sentence specification. PROT = VERB = GOAL = TOPIC = [N === DOC] [V === BITE] [TENSE === PAST] .[ADJ === OLD] [N === MAN] [PROT] 45 The functional feature is "TOPIC", and is specified as being the agent (or PROTago- nist) of the semantic structure. The feature value controls wether an active or passive sen- tence will be produced. The work reported in this paper extends …

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