The Use of Default Unification in a System of Lexical Types

In this paper we describe the encoding of a Unification-Based Generalised Categorial Grammar for English, in terms of a default inheritance network of types, implemented with YADU, which is an order independent default unification operation on typed feature structures. We then propose to use this framework to encode a Universal Grammar (UG) and associated parameters, following the Principles and Parameters Theory and describe how they are implemented. This is a clear and concise way of defining the UG with the parameters being straightforwardly defined in the type network, in a way which takes advantage of the default inheritance mechanism, to propagate information about parameters, throughout the lexical inheritance network. The parameters are set based on exposure to a particular language, and that is obtained by using a corpus of annotated sentences to set the parameters. The corpus was parsed and annotated using the implemented English grammar. The resulting formalisation is being used as the basis to perform studies about language acquisition.