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<Paper uid="J93-4001">
  <Title>The Interface between Phrasal and Functional Constraints</Title>
  <Section position="8" start_page="587" end_page="587" type="concl">
    <SectionTitle>
9. Concluding Remarks
</SectionTitle>
    <Paragraph position="0"> As we discussed in the introduction, the interleaved pruning strategy is substantially better than simple composition and so it is no surprise that it is a widely used and little questioned interface strategy. However, it is only one point in a complex and multi-dimensional space of possibilities, and not necessarily the optimal point at that. We outlined a number of alternative strategies, and presented preliminary measurements to suggest that factored extraction may give better overall results, although it is very sensitive to details of the grammar. Factored pruning also gives good results and is less sensitive to the grammar. The good results of these two strategies show how important it is to take advantage both of monotonicity and independence and of the polynomial nature of the phrasal constraints.</Paragraph>
    <Paragraph position="1"> The investigations summarized in this paper suggest several directions for future research. One direction would aim at developing a grammar compiler that automatically selects and moves the best set of features. A compiler could hide this transformation from the grammar developer or end user, so that it would be considered merely a performance optimization and not a change of linguistic analysis. Another research direction might focus on a way of adding functional pruning to the factored extraction algorithm so that it would be less sensitive to variations in the grammar.</Paragraph>
    <Paragraph position="2"> At a more general level, our explorations have illustrated the richness of the space of phrasal-functional interface possibilities, and the potential value of examining these issues in much greater detail. Of course, further experimental work using other grammars and larger corpora are necessary to confirm the preliminary results we have obtained. We also need more formal analyses of the computation complexity of interface strategies to support the intuitive characterizations that we have presented in this paper. We believe that the context-free nature of phrasal constraints has not yet been fully exploited in the construction of hybrid constraint processing systems and that further research in this area can still lead to significant performance improvements.</Paragraph>
  </Section>
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