Lexical Rules: What are they?

Horizontal redundancy is inherent to lexica consisting of descriptions of fnlly formed objects. This causes an unwelcome expansion of the lexical database and increases parsing time. To eliminate it, direct relations between descriptions of fnlly formed objects are often defined. These are additional to the (Typed Multiple) Inheritance Network which already structures the lexicon. Many implementations of horizontal relations, however, fail to generate lexieal entries on a needsdriven basis, so eliminate neither the problem of lexicon expansion nor that of inefficient parsing. Alternatively, we propose that lexical entries are descriptions of objects open to contextnal specification of their properties on the basis of constraints defined within the type system. This guarantees that only those grammatical lexical entries are infered that are needed for efficient parsing. The proposal is extremely modest, making use of only basic inference power and expressivity. 1 L e x i c a l R u l e s : w h a t are t h e y ? Within the strongly lexical framework of HPSG, lexi( 'al r u l e s are used to express relations among descriptions ..... a kind of indirect "horizontal relatedness" (Pollard & Sag 1987, 209) which can be contrasted with the vertical relations between the type(s) of lexical elements. Type relations are, of course, captured directly as the monotonic (typed) multiple inheritance network itself, which structures the lexicon. Typical examples of horizontal redundancy in the hierarchical lexicon thus conceived arc the Alternation phenomena (e.g. Dative Shift, the Locative Alternation, ctc) and word formation phenomena (inflectional and derivational morphology). In fact, Pollard and Sag also refer to declension class membership and similar facts as horizontM relations, and as we shall see, the boundary between vertical and horizontal relations is not immutably fixed once and for all. The notion of lexical rule is often given some status at the level of linguistic or psychological theory. (Pollard & Sag 1987) make reference to a generative or procedural interpretation of lexicM rules as a deductive mechanism which can be deployed on a needs only basis, for example, to generate words from a single base form. The conception of lexical rules as essential generative devices (rather than static statements expressing (sub-) regularities), is shared in much in:llnential work (e.g. (Bresnan 1982), (Pinker 1989)), although it is by no means universM, even within tIPSG. Viewed from an implcmentational perspective, onthe-fly application of lexicM rules brings with it a number of distinct advantages which follow from the drastic reduction in the size of the lexicM database (lexical construction is less time consmning and parsing time should be reduced as lexical look up is less ambiguous, etc). At first sight then it appears that the benefit of adding an external Lexical l(ule component outweighs the disadvantages (external powerNl mechanisms). We will first show that their role is less clear than this suggests and certainly more problematic, before suggesting in Section 2 an alternative which eschews any extra mechanisms. 1.1 Horizontal and Vertical Redundancy The parallel drawn above between vertical relatedness (expressed with the type system) and horizontM relatedness among descriptions of fully formed objects is however rather misleading. Monotonic multiple inheritance networks are most naturally used to represent generalisations over the properties that (groups of) linguistic objects