This chapter is concerned with the linguistic and formal foundations used for the competence-based performance model.
We first introduce constraint-based grammar theories as appropriate means for specifying reversible grammars. In these theories, the grammatical well-formedness of possible utterances is described in terms of identity constraints a linguistic structure must fulfill taking into account information of different strata (e.g., phonology, syntax and semantics) in a uniform and completely declarative way, e.g., Lexical Functional Grammar (LFG, [Bresnan1982]), Head-Driven Phrase Structure Grammar (HPSG, [Pollard and Sag1987]) and constraint-based categorial frameworks (cf. [Uszkoreit1986a] and [Zeevat et al.1987]).
Most important from a reversibility standpoint is that the theories only characterize what constraints are important during natural language use, not in what order they are applied. Thus they are purely declarative. Furthermore, since almost all theories assume that a natural language grammar not only describes the correct sentences of a language but also the semantic structure of grammatically well-formed sentences, they are perfectly well suited to a reversible system, because they are neutral with respect to interpretation and production.
The computational framework of our approach is based on constraint logic programming (CLP). CLP combines very well with the deductive view of language processing where parsing and generation are uniformly considered as proof strategies (cf. [Pereira and Warren1983], [Shieber1988] and chapter 4 of this thesis) as well as with the constraint-based view on current grammar theories. Moreover, we show in this thesis how a tight integration of parsing and generation can be realized using CLP in an elegant and efficient way.
These aspects together makes CLP an excellent platform for combining methods from Computational Linguistics and Artificial Intelligence and hence for achieving theoretically sound and practical natural language systems.
The rest of the chapter is organized as follows. In section 3.1 we informally present the basic ideas shared by all modern constraint-based grammar theories. Section 3.2 presents the formalism of [Höhfeld and Smolka1988] which is a general characterization of such constraint-based formalisms and an actual constraint language (called ) for representing linguistic structures. Although we have chosen a simple constraint language in order to highlight the new results in a clean but simple way, the generalization of the scheme guarantees that the results of this thesis also carries over for more complex constraint languages. In section 3.3 we show how the formalism is used for writing grammars and in section 3.4 we introduce the constraint-based view of natural language parsing and generation and discuss the relationship between ambiguities and paraphrases.
