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<Paper uid="P93-1016">
  <Title>PRINCIPLE-BASED PARSING WITHOUT OVERGENERATION 1</Title>
  <Section position="7" start_page="118" end_page="119" type="evalu">
    <SectionTitle>
6. Discussion and Related Work
</SectionTitle>
    <Paragraph position="0"> Complexity of unification The attribute vectors used here are similar to those in unification based grammars/parsers. An important difference, however, is that the attribute vectors used here satisfy the unil closure condition (Barton, Jr. et al., 1987, p.257). That is, non-atomic attribute values are vectors that consist only of atomic attribute values. For example: (7) a. ((cat v) +pas +whpg (wh-atts (cat p)) b. * ((cat v) +pas +ghpg (wh-atts (cat v) (np-att (cat n)))) (7a) satisfies the unit closure condition, whereas (7b) does not, because wh-atts in (7b) contains a non-atomic attribute np-atts. (Barton, Jr. et al., 1987) argued that the unification of recursive attribute structures is a major source of computational complexity. On the other hand, let a be the number of atomic attributes, n be the number of non-atomic attributes. The time it takes to unify two attribute vectors is a + na if they satisfy the unit closure condition. Since both n and a can be regarded as constants, the unification takes only constant amount of time. In our current implementation, n = 2, a = 59.</Paragraph>
    <Paragraph position="1"> Attribute grammar interpretation Correa (1991) proposed an interpretation of GB principles based on attribute grammars. An attribute grammar consists of a phrase structure grammar and a set of attribution rules to compute the attribute values of the non-terminal symbols.</Paragraph>
    <Paragraph position="2"> The attributes are evaluated after a parse tree has been constructed by the phrase structure grammar.</Paragraph>
    <Paragraph position="3"> The original objective of attribute grammar is to derive the semantics of programs from parse trees.</Paragraph>
    <Paragraph position="4"> Since programming languages are designed to be unambiguous, the attribution rules need to be evaluated on only one parse tree. In attribute grammar interpretation of GB theory, the principles are  encoded in the attribution rules, and the phrase structure grammar is replaced by X-bar theory and Move-~. Therefore, a large number of structures will be constructed and evaluated by the attribution rules, thus leading to a serious overgeneration problem. For this reason, Correa pointed out that the attribute grammar interpretation should be used as a specification of an implementation, rather than an implementation itself.</Paragraph>
    <Paragraph position="5"> Actor-based GB parsing Abney and Cole (1986) presented a GB parser that uses actors (Agha, 1986). Actors are similar to objects in having internal states and responding to messages. In our model, each syntactic category is represented by an object. In (Abney and Cole, 1986), each instance of a category is represented by an actor. The actors build structures by creating other actors and their relationships according to 0-assignment, predication, and functional-selection. Other principles are then used to filter out illicit structures, such as subjacency and case-filter. This generate-and-test nature of the algorithm makes it suscetible to the overgeneration problem.</Paragraph>
  </Section>
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