Appendix: Definitions of typed feature structures

The LinGO grammar consists of a specification of a type system and of various typed feature structures which are well-formed according to the type system. The typed feature structures function as grammar rules, lexical rules and lexical entries. There are several variant typed feature structure formalisms, with different computational properties, so in this appendix we very briefly specify the version assumed by the LinGO grammar. This appendix is necessarily terse and is only intended to allow a reader who already has a knowledge of typed feature structures to understand the specific formalism used in the LinGO grammar. The definitions given below basically follow Carpenter (1992), with the notion of type constraint from Copestake (1992). For formal details of typed feature structures in general see Carpenter (1992). A detailed account of the specific assumptions made here is given in Copestake (1999) (see Chapter 4 for an introduction and Chapter 5 for a semi-formal account). Note that the LinGO grammar uses a very restricted formalism. For instance, it does not utilize disjunctive feature structures, negation, implication, inequalities, defaults, set-valued features, extensionality or relational constraints. Constraint resolution does not require that every type be made maximally specific, and the type inference system is essentially non-recursive. The recursive power necessary in grammars is explicitly encoded via rules, which are expressed as typed feature structures, but interpreted as phrase structure rules.