Lexical-Functional Grammar: a Formal System For
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LexicalFunctional Grammar A Formal System for Grammatical Representation Ronald M Kaplan and Joan Bresnan In learning their native language children develop a remarkable set of capabilities They acquire knowledge and skills that enable them to pro duce and comprehend an indenite number of novel utterances and to make quite subtle judgments ab out certain of their prop erties The ma jor goal of psycholinguistic research is to devise an explanatory account of the mental op erations that underlie these linguistic abilities In pursuing this goal wehave adopted what wecallthe Competence Hypothesis as a metho dological principle Weassumethatanexplana tory mo del of human language p erformance will incorp orate a theoreti cally justied representation of the nativespeakers linguistic knowledge a grammar as a comp onent separate b oth from the computational mech anisms that op erate on it a processor and from other nongrammatical pro cessing parameters that might inuence the pro cessors b ehavior To a certain extentthevarious comp onents that we p ostulate can b e studied indep endently guided where appropriate bythewellestablished metho ds and evaluation standards of linguistics computer science and exp erimen tal psychologyHowever the requirement that the various comp onents ultimately must t together in a consistent and coherent mo del imp oses even stronger constraints on their structure and op eration This pap er originally app eared in The Mental Representation of Grammatical Rela tions ed Joan Bresnan Cambridge MA The MIT Press Kaplan ab gives an early version of the Comp etence Hyp othesis and discusses some ways in which the grammatical and pro cessing comp onents mightinteract Also see Ford Bresnan and Kaplan Formal Issues in LexicalFunctional Grammar edited by Mary Dalrymple Ronald M Kaplan John T Maxwell I I I Annie Zaenen c Copyright Stanford University Ronald M Kaplan and Joan Bresnan This pap er presents a formalism for representing the native sp eakers syntactic knowledge In keeping with the Comp etence Hyp othesis this formalism called lexicalfunctional grammar LFG has b een designed to serve as a medium for expressing and explaining imp ortant general izations ab out the syntax of human languages and thus to serveasa vehicle for indep endent linguistic research Of equal signicance it is a restricted mathematically tractable notation for which simple psycholog ically plausible pro cessing mechanisms can b e dened Lexicalfunctional grammar has evolved b oth from previous research within the transforma tional framework eg Bresnan and from earlier computational and psycholinguistic investigations Wo o ds Kaplan a Wanner and Maratsos The fundamental problem for a theory of syntax is to characterize the mapping b etween semantic predicateargument relationships and the sur face word and phrase congurations bywhich they are expressed This mapping is suciently complex that it cannot b e characterized in a simple unadorned phrase structure formalism a single set of predicateargument relations can b e realized in many dierent phrase structures eg ac tive and passive constructions and a single phrase structure can express several dierent semantic relations as in cases of ambiguity In lexical functional grammar this corresp ondence is dened in two stages Lexical entries sp ecify a direct mapping b etween semantic arguments and con gurations of surface grammatical functions Syntactic rules then iden tify these surface functions with particular morphological and constituent structure congurations Alternative realizations may result from alter native sp ecications at either stage of the corresp ondence Moreover grammatical sp ecications imp ose wellformedness conditions on b oth the functional and constituent structures of sentences The present pap er is concerned with the grammatical formalism it self its linguistic computational and psychological motivation are dealt with in separate pap ers In the next several sections weintro duce the formal ob jects of our theory discuss the relationships among them and dene the notation and op erations for describing and manipulating them Illustrations in these and later sections show p ossible LFG solutions to various problems of linguistic description Section considers the func tional requirements that strings with valid constituent structures must satisfy Section summarizes arguments for the indep endence of the con stituent functional and semantic levels of representation In Section weintro duce and discuss the formal apparatus for characterizing long distance grammatical dep endencies Weleave to the end the question of our systems generativepower Weprove in Section that despite their AFormal System for Grammatical Representation linguistic expressiveness lexicalfunctional grammars are not as p owerful as unrestricted rewriting systems Constituent structures and functional structures A lexicalfunctional grammar assigns two levels of syntactic description to every sentence of a language Phrase structure congurations are represented in a constituent structure A constituent structure or c structure is a conventional phrase structure tree a wellformed lab eled bracketing that indicates the sup ercial arrangementofwords and phrases in the sentence This is the representation on which phonological interpre tation op erates to pro duce phonetic strings Surface grammatical func tions are represented explicitly at the other level of description called functional structure The functional structure fstructure provides a precise characterization of such traditional syntactic notions as sub ject understo o d sub ject ob ject complement and adjunct The fstructure is the sole input to the semantic comp onent whichmay either translate the fstructure into the appropriate formulas in some logical language or provide an immediate mo deltheoretic interpretation for it Constituent structures are formally quite dierent from functional structures Cstructures are dened in terms of syntactic categories ter minal strings and their dominance and precedence relationships whereas fstructures are comp osed of grammatical function names semantic forms and feature symbols Fstructures and cstructures are also distinct from semantic translations and interpretations in which for example quantierscop e ambiguities are resolved By formally distinguishing these levels of representation our theory attempts to separate those gram matical phenomena that are purely syntactic involving only cstructures and fstructures from those that are purely lexical involving lexical en tries b efore they are inserted into cstructures and fstructures or se mantic for example involving logical inference Our framework thus facilitates an empirically motivated division of lab or b etween the lexical syntactic semantic and phonological comp onents of a grammar A cstructure is determined by a grammar that characterizes all p os sible surface structures for a language This grammar is expressed in a slightly mo died contextfree formalism or a formally equivalent sp eci cation such as a recursive transition network Wo o ds Kaplan For example the ordinary rewriting pro cedure for contextfree grammars would assign the cstructure to the sentence given the rules in a S NP VP b NP Det N c VP VNPNP Ronald M Kaplan and Joan Bresnan A girl handed the babya toy S NP VP Det N V NP NP Det N Det N A girl handed the baby a toy We emphasize that cstructure no des can b e derived only by phrase struc ture rules such as abc There are no deletion or movement op erations which could for example form the doubleNP sequence from a phrase structure with a to prep ositional phrase Suchmechanisms are unneces sary in LFG b ecause we do not map b etween semantically and phonolog ically interpretable levels of phrase structure Semantic interpretation is dened on functional structure not on the phrase structure representation that is the domain of phonological interpretation The functional structure for a sentence enco des its meaningful gram matical relations and provides sucient information for the semantic com p onent to determine the appropriate predicateargumentformulas The fstructure for would indicate that the girl noun phrase is the gram matical sub ject handed conveys the semantic predicate the baby NP is the grammatical ob ject and toy serves as the second grammatical ob ject The fstructure represents this information as a set of ordered pairs eachof which consists of an attribute and a sp ecication of that attributes value for this sentence An attribute is the name of a grammatical function or feature subj pred obj num case etc There are three primitive typ es of values a Simple symbols b Semantic forms that govern the pro cess of semantic interpre tation c Subsidiary fstructures sets of ordered pairs representing com plexes of internal functions A fourth typ e of value sets of symb ols semantic forms or fstructures is also p ermitted We will discuss this typ e when we consider the gram matical treatment of adjuncts AFormal System for Grammatical Representation Given p ossibility c an fstructure is in eect a hierarchyofat tributevalue pairs We write an fstructure by arranging its pairs verti cally inside square brackets with the attribute and value of a single pair placed on a horizontal line The following is a plausible fstructure for sentence spec a num sg subj pred girl tense past pred hand h subj obj obji spec the num sg obj pred baby spec a num sg obj pred toy In this