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<Paper uid="C90-2060">
  <Title>HOW TO INVERT A NATURAL LANGUAGE PARSER INTO AN EFFICIENT GENERATOR: AN ALGORITHM FOR LOGIC GRAMMARS</Title>
  <Section position="8" start_page="0" end_page="0" type="concl">
    <SectionTitle>
CONCLUSIONS
</SectionTitle>
    <Paragraph position="0"> In this paper we presented an algorithm for automatic inversion of a unification parser for natural language into an efficient unification generator. The inverted program of the generator is obtained by an off-line compilation process which directly manipulates the PROLOG code of the parser program. We distinguish two logical stages of this transformation: computing the minimal sets of essential arguments (MSEA's) for predicates, and generating the inverted program code with INVERSE. We have completed a first implementation of the system and used it to derive both a parser and a generator from a single DCG grammar for English (Strzalkowski and Peng, 1990).</Paragraph>
    <Paragraph position="1"> This method is contrasted with the approaches that seek to define a generalized but computationally expensive evaluation strategy for running a grammar in either direction without a need to manipulate its rules (Shieber, 1988), (Shieber et al., 1989), and see also (Colmerauer, 1982) and (Naish, 1986) for some relevant techniques, employing the trick known as goal freezing. To reduce the cost of the goal freezing, and also to circumvent some of its deficiencies, Shieber et al. (1989) introduce a mixed top-downPoottom-up goal expansion strategy, in which only selected goals are expanded during the top-down phase of the interpreter.</Paragraph>
    <Paragraph position="2"> This technique, still substantially more expensive than a fixed-order top-down interpreter, does not by itself guarantee that the underlying grammar formalism can be used bidirectionally, and it may need to be augmented by static goal reordering, as described in this paper.</Paragraph>
  </Section>
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