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<Paper uid="J92-2002">
  <Title>Inheritance and Constraint-Based Grammar Formalisms R6mi Zajac*t Project POLYGLOSS</Title>
  <Section position="5" start_page="175" end_page="176" type="concl">
    <SectionTitle>
4. Conclusion
</SectionTitle>
    <Paragraph position="0"> The TFS system has been developed to provide a computational environment for the design and the implementation of formal models of natural language. The TFS formalism is designed as a specification language that can be used to design and implement formal linguistic models. It is not a programming language: it does not offer means of defining control information that would make execution more efficient (but less general), as it would be needed if it would be envisaged to use the system in an application-oriented environment (e.g., as a parser in a natural language interface to a database system). From formal linguistic models developed in TFS, it could be envisaged to develop programs, i.e., parsers or generators, that would efficiently implement the declarative knowledge contained in the formal specifications. 13 13 See for example in Biggerstaff and Perlis (1989) the papers on the development of programs from specifications, a very important issue in software engineering. See also Ait-Kaci and Meyer (1990) for a programming language based on typed feature structures.</Paragraph>
    <Paragraph position="1">  R6mi Zajac Inheritance and Constraint-Based Grammar Formalisms The TFS system is implemented using rewriting techniques in a constraint-based architecture adapted to feature structures: * The language is a logical language directly based on typed feature structures, and supports an object-oriented style based on multiple inheritance.</Paragraph>
    <Paragraph position="2"> * Grammars are expressed as inheritance networks of typed feature structures. They define constraints on the set of acceptable linguistic structures. As a consequence, there is no formal distinction between &amp;quot;input&amp;quot; and &amp;quot;output.&amp;quot; * A unique general constraint solving mechanism is used. Specific mapping properties, based on constituency, linear precedence or functional composition, are not part of the formalism itself, but can be encoded explicitly using the formalism.</Paragraph>
    <Paragraph position="3"> Although the current implementation is very much at the level of an experimental prototype, and is still evolving, it has validated the basic concepts of the language and of the implementation, and has been used to test different linguistic models and formalisms such as LFG, DCG, HPSG, and SFG on small examples. From these various experimentations, we have defined extensions and improvements, both on the language and on the implementation, that are needed for scaling up the system. On the language side, more expressivity is needed. For example, disjunctions over feature structures, various kinds of negation (Ait-Kaci 1986), and sets of feature structures (Pollard and Moshier 1990) are necessary to formalize, e.g., nontrivial semantic structures. Some types together with a specific syntax and associated operations could be conveniently added to the system as libraries of built-in types, e.g., characters, strings, and trees.</Paragraph>
    <Paragraph position="4"> On the implementation side, the use of implementation techniques adapted from PROLOG implementations, constraint satisfaction languages, and object-oriented languages can be beneficial to the implementation of typed feature structure-based systems and have to be more thoroughly explored.14One of the major efficiency issues in the current implementation is the lack of an efficient indexing scheme for typed feature structures. For example, since the dictionaries are accessed using unification only, each entry is tried one after the other, leading to an extremely inefficient behavior with large dictionaries. Thus, the use of a general indexing scheme based on a combination of methods used in PROLOG implementations and in object-oriented database systems is necessary and will be implemented in a future version of the system.</Paragraph>
  </Section>
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