Default Unification Research Articles

Deterministic shift-reduce parsing for unification-based grammars

AbstractMany parsing techniques assume the use of a packed parse forest to enable efficient and accurate parsing. However, they suffer from an inherent problem that derives from the restriction of locality in the packed parse forest. Deterministic parsing is one solution that can achieve simple and fast parsing without the mechanisms of the packed parse forest by accurately choosing search paths. We propose new deterministic shift-reduce parsing and its variants for unification-based grammars. Deterministic parsing cannot simply be applied to unification-based grammar parsing, which often fails because of its hard constraints. Therefore, this is developed by using default unification, which almost always succeeds in unification by overwriting inconsistent constraints in grammars.

Maximal Consistent Subsets

Default unification operations combine strict information with information from one or more defeasible feature structures. Many such operations require finding the maximal subsets of a set of atomic constraints that are consistent with each other and with the strict feature structure, where a subset is maximally consistent with respect to the subsumption ordering if no constraint can be added to it without creating an inconsistency. Although this problem is NP-complete, there are a number of heuristic optimizations that can be used to substantially reduce the size of the search space. In this article, we propose a novel optimization, leaf pruning, which in some cases yields an improvement in running time of several orders of magnitude over previously described algorithms. This makes default unification efficient enough to be practical for a wide range of problems and applications.

Fixed segmentism and dual description

This article considers the theoretical challenge posed by fixed segmentism, the introduction of a particular segment or segments into a reduplicated string. The languages of the Indian subcontinent contain many examples of this type and a wide range of different patterns is presented. Previous accounts of fixed segmentism are surveyed and applied to these data, with particular attention to the optimality theory approach of Alderete et al. (1999). This is contrasted with a new proposal, termed “dual description”, which is set within the framework of declarative phonology. This uses the mechanism of default inheritance to implement the repetition of phonological material from the base. The introduction of fixed segments is handled by default unification, a procedure that conjoins the inherited material with information specific to the reduplicant, giving priority to the latter. This approach is applied to the Indian data and shown to handle the full range of patterns of fixed segmentism successfully. Its potential as a general theory of reduplication is also demonstrated by applying dual description to various other kinds of reduplicative phenomena.

Gender mismatches in Spanish and French N1/A de N2 affective constructions

I examine Spanish and French agreement in sentences with "affective" N/A de N constructions, in terms of an agreement theory growing out of Pollard and Sag (1994, §2) and Kathol (1999), with a distinction between two kinds of agreement relations: index agreement and morphosyntactic concord. The application of this theory to hybrid nouns (Wechsler and Zlati´c, 2000) extends straightforwardly to affective constructions. Furthermore, Kathol's characterization of the difference between hybrid nouns in Spanish and French, which I pair with an interpretation in terms of the default unification mechanism of Lascarides and Copestake (1999), turns out to make correct predictions about subtle differences in predicate agreement with affective constructions in the two languages.

Order independent and persistent typed default unification

We define an order independent version of default unification on typed feature structures. The operation is one where default information in a feature structure typed with a more specific type, will override default information in a feature structure typed with a more general type, where specificity is defined by the subtyping relation in the type hierarchy. The operation is also able to handle feature structures where reentrancies are default. We provide a formal semantics, prove order independence and demonstrate the utility of this version of default unification in several linguistic applications. First, we show how it can be used to define multiple orthogonal default inheritance in the lexicon in a fully declarative fashion. Secondly, we show how default lexical specifications (introduced via default lexical inheritance) can be made to usefully ‘persist beyond the lexicon’ and interact with syntagmatic rules. Finally, we outline how persistent default unification might underpin default feature propagation principles and a more restrictive and constraint-based approach to lexical rules.