A Deterministic Syntactic-Semantic Parser

We consider t ha t we have made a dec is ive step towards determinism in pa rs ing . We agree w i th Winograd 's h e s i t a t i o n to evaluate the determinism hypothes i s as formulated by Marcus. However, t h i s does not make us doubt about the p o s s i b i l i t y of de te rmi nism; on the con t ra ry , we examined not only how to improve over Marcus, but also the h i s t o r i c a l reasons of the non-deteminism of most systems. Our improvements are based on two p r i n c i p l e s : syntac t ic -semant ic i n t e g r a t i o n , and quas i s imu l t a ,ieousness. The f i r s t means t h a t there is no such t h i n g as " the autonomy of syntax" (Marcus); so, we agree w i th Schank and, f u r t h e r , we showed tha t local semantic ambigu i t ies could be solved d e t e r m i n i s t i c a l l y (Marcus (ch.10) claims t ha t these ambigu i t ies need p a r a l l e l p rocess ing) . The second permits the processing of s t ruc tu res too d i f f i c u l t f o r PARSIFAL e .g . l o c a l l y ambiguous PP attachments. Deta i led examples support our proposals. I POSSIBLE FACTORS OF NON-DETERMINISM With the notable except ion of Marcus, already mentioned , most "parser makers" seem to agree on having a non-de te rm in is t i c s t ra tegy (backt rack ing or p a r a l l e l i s m ) . Schank (e t a l . ) have always perceived the inadequacy of those s t r a t e g i e s , and practised the i n t e g r a t i o n of syntax and semantics, but they have conserved a top-down approach ( ' p r e d i c t i o n ' cf Wilks) We shall t ry to show the importance of 4 factors of nondeterminism which are not related to NL ambiguity: the architecture of systems, ru les and i n t e r p r e t e r , stack mecanism, word by word processing. A. A rch i tec tu re of systems Decision elements f o r a parser are of 4 k inds : morphosyntact ic, semantic, pragmatic and con tex tua l . So, i t is obvious tha t cons ider ing only one of them br ings about non-determinism, independently of NL. i t s e l f . However, even when a system uses a l l of t h i s i n f o r m a t i o n , non-determinism may appear due to an a r t i f i c i a l separat ion between modules (eg syn tac t i c & semant ic) . Some non-determinism remains even in the most sophisticated modular systems and t h a t ' s because: -one module is dominating the others (as in SHRDLU) t o o many i n t e r a c t i o n po in ts e x i s t t h e non-det . may f i n d refuge in one of the modules (as in PARSIFAL, the semantic one). B. Representat ion f o r a grammar and i n t e r p r e t o r The i n t e r p r e t e r has usua l l y e i t h e r a top-down or a bottom-up s t ra tegy , but most f requen t l y top-down. But a r e l a t i v e c lause, f o r ins tance, should ra ther be parsed bottom-up, in order to be attached de te r m i n i s t i c a l l y . In theory , the Marcus parser seems to permi t t h i s s t ra tegy ( " a t t e n t i o n s h i f t i n g r u l e s " ) . But the grammar Marcus a c t u a l l y wrote does not take advantage of t h i s f a c i l i t y \nd r e l a t i v e clauses are a lso parsed top-down. From h is t h e o r e t i c a l work , we s t i l l have the con f i rmat ion o f f o l l o w i n g p o i n t : A completely top-down or completely bottom-up mecanism must parse non d e t e r m i n i s t i c a l l y some sentences tha t would not g ive the same t roub le to a mixed s t ra tegy . ( I n the f i e l d of programming languages, such a mixed s t ra tegy has already been app l i ed , Ear ley) Now, another important p o i n t : in a t r a d i t i o n a l dec la ra t i ve system (ATN or PSG), the ru les are independent of the i n t e r p r e t e r . So, when a c o n f l i c t arises between two v a l i d r u l e s , i t i s not poss ib le f o r the i n t e r p r e t e r to make a dec i s i on . Hence,one has to l i m i t the r o l e of the i n t e r p r e t e r and the ru les must be non ambiguous ( t h e i r cond i t i on par t being as det a i l e d as p o s s i b l e ) .