<?xml version="1.0" standalone="yes"?>
<Paper uid="P90-1027">
  <Title>AUTOMATED INVERSION OF LOGIC GRAMMARS FOR GENERATION</Title>
  <Section position="4" start_page="212" end_page="212" type="relat">
    <SectionTitle>
RELATED WORK
</SectionTitle>
    <Paragraph position="0"> The idea that a generator for a language might be considered as an inverse of the parser for the same language has been around for some time, but it was only recently that more serious attention started to be paid to the problem. We look here only very briefly at some most recent work in unificatlon-hased grammars. Dymelman and Isabelle (1988) address the problem of inverting a definite clause parser into a generator in context of a machine translation system and describe a top-down interpreter with dynamic selection of AND goals 1 (and therefore more flexible than, say, left-to-right interpreter) that can execute a given DCG grammar in either direction depending only upon the binding status of arguments in the top-level literal. This approach, although conceptually quite general, proves far too expensive in practice.</Paragraph>
    <Paragraph position="1"> The main source of overhead comes, it is pointed out, from employing the nick known as goal freezing (Colmerauer, 1982; Naish, 1986), that stops expansion of currently active AND goals until certain variables get instantiated. The cost, however, is not the only reason why the goal freezing techniques, and their variations, are not satisfactory. As Shieber et al.</Paragraph>
    <Paragraph position="2"> (1989) point out, the inherently top-down character of goal freezing interpreters may occasionally cause serious troubles during execution of certain types of recursive goals. They propose to replace the dynamic ordering of AND goals by a mixed topdown/bottom-up interpretation. In this technique, certain goals, namely those whose expansion is defined by the so-called &amp;quot;chain rules &amp;quot;2, are not expanded during the top-down phase of the interpreter, but instead they are passed over until a nearest non-chain rule is reached. In the bottom-up phase the missing parts of the goal-expansion tree will be filled in by applying the chain rules in a backward manner. This technique, still substantially more expensive than a fixed-order top-down interpreter, does not by itself guarantee that we can use the underlying grammar formalism bidirectionally. The reason is that in order to achieve bidirectionality, we need either to impose a proper static ordering of the &amp;quot;non-chain&amp;quot; AND * Literals on the right-hand side of a clause create AND goals; llterals with the same predicate names on the left-hand sides of different ehuses create OR goals.</Paragraph>
  </Section>
class="xml-element"></Paper>