Tree Unification Grammar

INTRODUCTION There is a mind in uni~ca~on-based grammar formalisms towards using a single grammar stmctme to contain the phonological, syntactic and semantic information associated with a linguistic expression. Adopting'the terminology used by Pollard and Sag (1987), this grammar structure is called a sign. Grammar rules, guided by the syntactic information contained in signs, are used to derive signs associated with complex expressions from those of their constituent expressions. The relationship between the signs and the complex signs derived from grammar rule application can be expressed in derivationai structures. These structures both explicitly illustrate relations that are implicit in the syntax of the signs and express relations that are present in the grammar roles. Tree unification grammar (TUG) is a formalism which uses function-argument (FA) specif~ationa as its primary grammar structures. These specifications resemble partially specified derivational stmcmn~ of sign-based formalisms like head-driven phrase structure grammar (HPSG) (Pollard and Sag, 1987) and unification categorial grammar (UCG) (7_,eevat, Klein and Calder, 1987). TUG uses FA specifications as lexical entries and possesses a single grammar rule which combines these specifications to obtain a specification for the complex expression being analysed. The use of FA specifications allows generafisations that are often captured in grammar rules to be captured in the lexicon. MOTIVATION The development of TUG was a consequence of investigating extensions to the UCG framework. As described by Zeevat, Kh~,, . -d Calder (1987), UCG is a grammar formalism which combines SOme of the notiow~s of categorial grammar with those of unification-based formalisms like HPSG and PATR-II (Shicber el.at., 1983). The nsse.t~h .~tM~,,~d in this lmq~r w u ~ o~ at the Univmlity of EdlnbeJqth under the rapport of • BrifiJh C,~-, '~weallh Scholmhlp and at 51hUm FmJ~ U u i ~ k y unde* ms Advmu~ Synmll Imt i~e ~ Fellov~hip. Special thar, Jo to the Omm: f ~ Systmm Scknm md zhe L*bm.atm.y for ~ r md R n e m ~ at Simon Fruer Unlve~izy fro. additkmal ml~pe~ I would I.'%-.. to t/rank Dm~ P~ md Om ACL mvi~a for thmt ¢,omm~B ,~4 m I l ~ Like HPSG, the fundamental construction used in UCG is the sign. A UCG sign has auributes for phonology, category, semantics and order. Consider the sign for the expression Mary walks shown in (I). (I) Mary-walks smt[f'm] [eli [[fllmary(fl), [el]walk(el,fl)] The phonology attribute of this sign (ie. Mary-walks) represents a phonological specification of the linguistic expression associated with the sign. For our needs we will use a simple sequence of words separated by hyphens. The category structure of a sign is very similar to that used by categorial grammar. There are three primitive categories, namely sent, np, and noun. Complex categories are of the form A / B, where B is a sign and A is a category (either primitive or complex). The semantic representation uses a language called InL (Zcevat, Klein and Calder 1987) which incorporates many of the features of discourse ~p, csentation theory (Kamp 1981). An InL formula is of the form [a]Condition where Condition consists of a predicate name followed by its argument list. Each element of the argument list is either a variable (ie. discourse marker) or an InL formula. The variable a preceding Condition is the index of the fonnnla. The order attribute of a sign contains information which is used to determine the ordering of the phonology of components during rule application. If an argument possesses pre as its order, then the phonology of the functor precedes that of the argument in that of the msuh. The value post describes the opposite situation. There is no restriction on the order of (1) as indicated by the appearance of the 'don't care' variable '_ ' in the order attribute. InL variables are assigned sorts. A sort can be thought of as a collection of features based on factors like gender and number. Unification of variables of incompatible sons will fail, thus providing a mechanism by which semantic information can restrict possible derivations. There are different sons for events, states and objects. Variables of the object sorx may be further specified with respect to gender (masculine, feminine, or neuter), and number. Unsorted variables will be denoted by the leuer a, events by e, states by s, and gendedess objects by x, y, and z. The letter m will be used to represent variables corresponding to a masculine object, f for feminine, and n for neuter. Unique identifiers which will be used to distinguish variables will appear as numbers following the variable names (ie. nl, ml, s2). Signs may be underspecified and through the application of the grammar rules they may become increasingly specified by the merging of information. Only two grammar rules are proposed in ('Zeevat, Klein and Calder, 1987): (2) Wt-W2: C: S: ~ Wt: C4(W2:C2:S2:pre): S: _,