A Symmetrical Approach to Parsing and Generation

Lexicat Grammars are a class of unification grammars which share a fixed rule component, for which there exists a simple left-recursion elimination transformation. The parsing and generation programs ale seen as two dual non-left-recursive versions of the original grammar, and are implemented through a standard top-down Prolog interpreter. Formal criteria for termination are given as conditions on lexical entries: during parsing as well as during generation the processing of a lexical entry constimes some amount of a guide; the guide used for parsing is a list of words remaining to be analyzed, while the guide for generation is a list of the semantics of constituents waiting to be generated. I . I n t r o d u c t i o n S y m m e t r y b e t w e e n p a r s i n g and generat ion . There is a natural appeal to the attempt to characterize parsing and ge~;era~ion in a symmetrical way. This is because the statement of the problem of revers ibi l i ty is naturally synlmetr ical : parsing is concerned with recovering semantic content from phonological content, generation phonological content from semantic content. It has been noted by several researchers ([$88], tN891, [SNMP891) that certain problems (left-recursion) and techniques (left-corner processing, linking, Ear!ey deduction) encountered in the parsing domain hJ, 'e o,rrelates in the generation domain. It is then na!:ural to wy and see parsing and generation as instances of a single paradigm; [$881 and [D[88, DI90I are attempts in this direction, but are hindered by the fact that there is no obvious correlate in gene,'ation of the string indexing techniques so prominent in parsing {string indices in chart parsing, differential lists m DCG parsing). Guides . What we propose here is to take a step back .and abstract file notion of string index to that of a ;,¢¢iUc. This gci~er,d notion ,,viii apply to both parsing aud generation, but it wi/! be instantiated differently in the va'o modes. The purpose of a guide is to orient the proof procedure , speci f ic to ei ther parsing or generation, in such a way that: (i) the guide is initialized as a direct function of the input (the string in parsing, thc semantics in generation), (it) the current stale of the ,~uide strongly constrains the next access lo the lexicon, (iii) after lexical access, the size of the guide strictly decreases (,gMde-consumption co~lditic.1, see section 3). Once a guide is specified, the generation problem (respect ively the parsing problem I) then reduces to a problem fornml!y simihtr to the problem of parsing v, ith a DCG [PW80} containing no empty p r o d u c t i o n s 2 (ie rules whose right-hand side is the empty string []). Several parsing techniques can be applied to this problem; we will be concerned here with a top-down parsing approach directly implementable through a standard Prolog interpreter. This approach relies on a lefi-recl~r,sioll-climination trans/brmation for a certain class of definite clause programs (see section 33. The ability 1o specify guides, for parsing or for generat ion, depends on certain compos i t iona l i ty hypotheses which the underlying grammar has to satisfy. I Thb, hall of the statcmenl ma> seem tautological, but it is not: see the attempt :it a reinlerprctalion of left exirap~sition iri terms of guides in section 5. 2 Al~o <'ailed meh' r.h 'x I11781. H y p o t h e s e s on c o m p o s i t i o n a H i t y . The parsing and general ion problems can be rendered t ractable only if certain hypmheses are made concerning the composit ion of linguistic structures. Thus generation can be arduous if the semantics associated with the composition of two structures is the nm'estricted lambda-application 3 of tile first structure's semantics on the second structure's semantics: this is because knowledge of the mother's semantics does not constrain in a usable way the semantics of the d a u g h t e r s . 4 On the contrary, parsing is greatly simplified if the string associated with the composition of two strqctures is the concatenation of tile strings associated with each st,ucture: one can then use string indexing to orient and control tl'e progression of the parsing process, as is done in DCG under tile guise of "dil'ferential lisls". l , e× ica l G r a n l m a r . The formalism of Lexical Grammar (LG) makes explicit certain compositionality hypotheses which ensure the existence of guides for parsing as well as for generation. A Lexical Grammar has two parts: a (variable) lexicon and a (fixed) rule component . The rule component, a definhe clause specification, spells out basic linguistic compositionality rules: (i) how a wellformed linguistic structure A is composed from wellformed structures B and (27: (it) what .:ire the respective statuses of B and C (left consti tuent vs ri,,,ht constituent, syntactic head vs syntactic dependenl , semantic f-wad vs semantic depemlent): and (iii) how the string (,'esp. semantics, subcategorizat ion list . . . . ) associated with A is related to the strinoA (resp. semantics, subcategorization lists . . . . ) associated with /3 and C (see sectioi, 2). The ability to define a guide for parsing is a (simple) consequence of the fact that the string associated with A is the concatenation of the strings associated with B and (.,5. The ability to define a guide for generation is a (less simple) consequence of LG's hypotheses on subcategorization (see sections 2 and 4). "~ By tmrestricted lambda-application, we mean functional application lbtlowed by, ivwriting to a ilOl'tlla] lollll, 4 In theories favoring such an approach (such as GPSG IGKPS871), parsing may be computatiollally tractable, but generation does not seem to be. These theories can be questioned as plausible computational models, for they should be judged on Iheir ability to account for production behavior (generation) as well as for understanding behavior {parsing). 5 A fairly standard assumption, ll: empty string lealizalions are allowed, then extraposifion call still be handled, as '~ketched in section 5.