Lexical Rules In Constraint-Based Grammars

Lexical rules have been used to cover a very diverse range of phenomena in constraint-based grammars. Examination of the full range of rules proposed shows that Carpenter’s (1991) postulated upper bound on the length of list-valued attributes such as subcat in the lexicon cannot be maintained, leading to unrestricted generative capacity in constraint-based formalisms utilizing HPSG-style lexical rules. We argue that it is preferable to subdivide such rules into a class of semi-productive lexically-governed genuinely lexical rules, and a class of fully productive unary syntactic rules. We develop a restricted approach to lexical rules in a typed default feature structure (TDFS) framework (Lascarides et al., 1996; Lascarides and Copestake, 1999) which has enough expressivity to state, for example, rules of verb diathesis alternation, but which does not allow arbitrary manipulation of list-valued features. An interpretation of such lexical rules within a probabilistic version of a TDFS-based linguistic (lexical and grammatical) theory allows us to capture the semi-productive nature of genuinely lexical rules, steering an intermediate course between fully generative or purely abbreviatory rules. We illustrate the utility of this approach with a treatment of dative constructions within a linguistic framework which borrows insights from the constraint-based theories: HPSG, UCG (Zeevat et al., 1987) and Construction Grammar (Goldberg, 1995). We end by outlining how our approach to lexical rules allows for a treatment of passive and recursive affixation, which are generally assumed to require unrestricted list manipulation operations.