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<Paper uid="P93-1029">
  <Title>F-PATR: FUNCTIONAL CONSTRAINTS FOR UNIFICATION-BASED GRAMMARS</Title>
  <Section position="9" start_page="220" end_page="220" type="concl">
    <SectionTitle>
7. CONCLUDING REMARKS
</SectionTitle>
    <Paragraph position="0"> One of the goals of this paper is to bring the work of Ait-Kaci and Nasr to the attention of the computational linguistics community. Their techniques for marrying declarative and functional programming paradigms are an important avenue to explore in expanding the expressiveness of formalisms for linguisic applications. The design issues encountered in building an implementation of F-PATR should be of interest to implementors of such a paradigm. Of course we do not address here issues in the logic of such feature structures or their declarative semantics. The significant differences of F-Patr from Le Fun include an alternative approach to dereferencing certain data types, a change motivated by an environment in which parsing control is outside the unification process, and also an extension to a simple form of disjunction. In contrast to the research projects that implement unification-based grammar formalisms on top of Prolog, this implementation has built a unification environment on top of Lisp. The job of integrating the declarative and functional paradigms is made considerably easier by relying on Lisp for lambda conversion and function evaluation.</Paragraph>
    <Paragraph position="1"> In the by now extensive literature on unification grammar frameworks, the current proposal figures as a somewhat conservative, and yet radically expressive, extension to PATR-II. It is conservative in that the logic of feature structures includes only minimal disjunction and no negation or conditionalization. But the extension leads to unlimited expressive power by bringing in the full power of function evaluation. It appears to be an extension appropriate for the representational problems we encountered, but it also has led to unanticipated uses. For example, in writing the semantics for graphical grammars we have been able to use functions in feature structures as a way of building forms that can simply be evaluated to invoke the appropriate operations for applications. Here again, having more control over when evaluation takes place external to the unification process has proved to be important.</Paragraph>
    <Paragraph position="2"> There are limitations, however, to the expressive power of F-PATR as it stands. It cannot directly support some of the constraints envisioned in current HPSG literature, for example, because of F-PATR's restrictions on arguments to functional constraints. In HPSG, relations constrain not just atomic values but also general feature structures incuding lists and sets.</Paragraph>
    <Paragraph position="3"> Such an extension to F-PATR is not planned by the author but it may be of interest. From the logic grammar point of view, the work reported on here may be relevant as a source of ideas for efficiency. Constraints expressed as relations in frameworks such as Zajac (1992) could instead be expressed in F-PATR as compiled functions, leading perhaps to improved runtime speeds.</Paragraph>
    <Paragraph position="4"> The MCC/Bellcore implementation of F-PATR includes both destructive and nondestructive versions of unification. The destructive version is, as expected, more straightforward to implement but more expensive computationally given that over copying and early copying are profligate (see Wroblewski 1987). The algorithms for nondestructive unification have been influenced by Tomabechi (1991), but applicative expressions and residuations change the landscape significantly. There tends to be extensive circularity in the data structures: residuated argument nodes point to predicates that in turn point back to their arguments; residuations in applicative-valued nodes point to unification forms that in turn point back to the applicative nodes. There is a need for future work to address issues of space and time efficiency for extensions represented by F-PATR just as there has been such a need for other PATR-II extensions.</Paragraph>
    <Paragraph position="5"> A line of research that the author is pursuing currently (Wittenburg 1992b) is to design a more specialized grammar formalism that finesses some of the complexity of residuation and unification through a version of &amp;quot;pseudo-unification&amp;quot; (Tomita 1990). In contrast to residuation, which manages function evaluation at runtime, the idea is to manage the order of evaluation for functional constraints at compile time.</Paragraph>
    <Paragraph position="6"> In grammar formalisms and parsers under investigation, it is possible for a compiler to order constraints within rule data structures such that evaluation readiness is a deterministic matter, circumventing the need for runtime checks and extra data structures required for delaying evaluation dynamically.</Paragraph>
  </Section>
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