16.6.1 Advanced: Extensions to Typing

DEFAULTS

Typed feature structures can be extended by allowing for inheritance with defaults. Default systems have mainly been used in lexical type hierarchies of the sort described in the previous section, in order to encode generalizations and subregular exceptions to them. In early versions of default unification the operation was order

dependent, based on the priority union operation (Kaplan, 1987). More recent architectures are order-independent (Lascarides and Copestake, 1997; Young and Rounds, 1993), related to Reiter's default logic (Reiter, 1980).

Many unification-based theories of grammar, including HPSG (Pollard and Sag, 1987, 1994) and LFG (Bresnan, 1982) use an additional mechanism besides inheritance for capturing lexical generalizations: the lexical rule. Lexical rules (Jackendoff, 1975) express lexical generalizations by allowing a reduced, hence more redundancy-free lexicon to be automatically expanded by the rules. See Pollard and Sag (1994) for examples, Carpenter (1991) on complexity issues, and Meurers and Minnen (1997) on efficient implementation. Conversely, see Krieger and Nerbonne (1993) on using the type hierarchy to replace lexical rules.

Types can also be used to represent constituency. Rules like (16.12) on page 15 used a normal phrase structure rule template and added the features via path equations. Instead, it's possible to represent the whole phrase structure rule as a type. In order to do this, we need a way to represent constituents as features. One way to do this, following Sag and Wasow (1999), is to take a type phrase which has a feature called DTRS (“daughters”), whose value is a list of phrases. For example the phrase I love New York could have the following representation, (showing only the DTRS feature):