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<Paper uid="P98-2227">
  <Title>Head-Driven Generation with HPSG</Title>
  <Section position="3" start_page="0" end_page="0" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> A natural approach to generation with Head-driven Phrase Structure Grammar (Pollard and Sag, 1994) is to use a head-driven algorithm. HPSG is head-driven not only syntactically, but also semantically. While the Head Feature Principle requires identity of major syntactic features between a phrase and its syntactic head daughter, the Semantics Principle (in various formulations) requires identity of major semantic features between a phrase and its semantic head daughter. Since the semantic head is very clearly defined in HPSG, semantic head-driven generation should be easy to implement.</Paragraph>
    <Paragraph position="1"> Efficient head-driven generation algorithms, such as BUG, SHD and CSHD, have been presented as Prolog algorithms for use with DCG grammars. In Section 2 we briefly describe how an HPSG grammar can be implemented as a PSG with typed feature structures, which can be compiled into a DCG by the ProFIT system. In this way, HPSG grammars can be used with the existing Prolog algorithms.</Paragraph>
    <Paragraph position="2"> Such a combination of head-driven grammar and head-driven generator works well if the semantics is strictly head-driven. However, in Section 3 we show that if we implement the HPSG textbook semantics, with quantifier storage and contextual background conditions, the notion of semantic head becomes unclear and this approach no longer works. In fact, head-driven generation of even simple phrases such &amp;quot; Visiting researcher of Sharp Corporation, Japan. as &amp;quot;Kim walks&amp;quot; (Chapter 1 of the HPSG textbook) raises fundamental difficulties.</Paragraph>
    <Paragraph position="3"> To use a semantic head-driven algorithm, we must adopt recent HPSG proposals to put quantifier store and contextual background inside semantic heads.</Paragraph>
    <Paragraph position="4"> We summarize these proposals in Section 4, and show how they can be implemented in the ProFIT HPSG grammar. We conclude that head-driven generation with HPSG is possible, but there are some difficulties in implementing this approach.</Paragraph>
  </Section>
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