Section 11.3 Features structures in the Grammar ─ Jin Wang

How to specify a way to integrate feature structures and unification operations into the specification of a grammar? feature structures unification Augmenting the rules of ordinary context-free grammars with attachments that specify feature structures for the constituents of the rules, along with appropriate unification operations that express constraints on those constituents.

β 0 ─> β 1... Β n (PATR- Ⅱ system, Shieber 1986) {set of constraints} The specified constraints have one of the following forms: ﹤ β i feature path ＞ = Atomic value ﹤ β i feature path ＞ = ﹤ β j feature path ＞

How to use these constraints??? At the beginning of this chapter: S ─> NP VP Only if the number of the NP is equal to the number of the VP Using the new notation: S ─> NP VP ﹤ NP NUMBER ﹥ = ﹤ VP NUMBER ﹥ So, the simple generative nature of context-free rules has been fundamentally changed by this augmentation.

Unification constraints can be applied to four interesting linguistic phenomena: Agreement Grammatical heads Subcategorization Long-distance dependencies. subject-verb agreement Agreement S ─> NP VP ﹤ NP AGREEMENT ＞ = ﹤ VP AGREEMENT ＞ - This flight serves breakfast. -Does this flight serve breakfast? -Do these flight serve breakfast? S ─> Aux NP VP ﹤ Aux AGREEMENT ＞ = ﹤ NP AGREEMENT ＞ - Does this flight serve breakfast? - Do these flight serve breakfast?

Determiner-nominal agreement Agreement NP ─> Det Nominal ﹤ Det AGREEMENT ＞ = ﹤ Nominal AGREEMENT ＞ ﹤ NP AGREEMENT ＞ = ﹤ Nominal AGREEMENT ＞

Non-lexical grammatical constituents can acquire values for at least some of their features from their component constituents. VP ─> Verb NP = Verb ─> serves = SG = 3 The same technique works for the remaining NP and Nominal categories.

Head Features VP ─> Verb NP =<Verb AGREEMENT> NP ─> Det Nominal =<Nominal AGREEMENT> =<Nominal AGREEMENT> Nominal─> Noun =<Noun AGREEMENT> VP ─> Verb NP = NP ─> Det Nominal = =<Nominal HEAD AGREEMENT> Nominal─> Noun = The features for most grammatical categories are copied from one of the children to the parent. Head of the phrase: The child that prvides the features. Head features: The features that are copied.

Subcategorization Verbs can be picky about the pattern of arguments they will allow themselves to appear with. SUBCAT: An atomic feature. A proper way to introduce feature structures to distinguish among the various members of the verb category. Verb ─> serves = SG = TRANS

This constraint is enforced by adding corresponding constraints to all the verb phrase rules in the grammar: VP ─> Verb = = INTRANS VP ─> Verb NP = = TRANS VP ─> Verb NP NP = = DITRANS However, this approach is somewhat opaque since these unanalyzable SUBCAT symbols do not directly encode either the number or type of the arguments that the verb expects to take.

A somewhat more elegant solution: - One argument (Serves dinner ) Verb ─> serves = SG = NP = END - Two arguments (leaves Boston in the morning ) Verb ─> serves = SG = NP = PP = END

Those examples just show rather simple subcategorization structures for verbs. In fact, verbs can subcategorize for quite complex subcategorization frames. Also, the notion of subcategorization can be used in other parts of speech, such as the prepositions.

Which flight do you want me to have the travel agent book ? Long-Distance Dependencies Sometimes, a constituent subcategorized for by the verb is not locally instantiated, but is in a long-distance relationship with the predicate. What cities does Continental service ?

Many solutions to representing long-distance dependencies in unification grammars involve keeping a gap list, implemented as a feature GAP, which is passed up from phrase to phrase in the parse tree.