T H E P A R A L L E L EXE~ERT P A R S E R
(PEP):
A T H O R O U G H L Y %~IVISED DESCENDAN'~ O F THE W O R D E X P E R T PARSEI% (WEP)
M. D E V O S Katholieke Universiteit Leuven Campus Gasthuisberg N e u r o p h y s i o l o g y Lab H e r e s t r a a t 49 B - 3 0 0 0 Leuven, B e l g i u m
G. A D R X A E N S Siemens NLP Research & Katholieke Universiteit Leuven M. T h e r e s i a s t r a a t 21 B - 3 0 0 0 Leuven, B e l g i u m ( s i e g e e r t @ k u l e s . u u c p or
[email protected])
Y.D. W I L L E M S Katholieke Universiteit Leuven D e p a r t m e n t of C o m p u t e r S c i e n c e Celestijnenlaan 200A B - 3 0 3 0 Heverlee, B e l g i u m (
[email protected])
~H>st z'act
in t h i s p a p e r we p r e s e n t PEP (the P a r a l l e l Expert Parser~ Devos 198'7), a radically revised descendant of WEP (the W o r d E x p e r t Parser, Small 1980) . WEP's idea of l i n g u i s t i c e n t i t i e s as i n t e r a c t i n g processes has b e e n retained, b u t its a d h e r e n c e to the w o r d as the only entity has b e e n rejected. Experts exist at different levels, communicate through rigidly defined protocols and are now fully designed to run in parallel. A p r o t o t y p e of PEP is implemented in F l a t C o n c u r r e n t P r o l o g ar{d runs in a L o g i x environment.
2. T h e W o E d E x p m r t P a r s e r b~iefly described
Io I n t r o d u c t i o n
Work on understanding
parallel natural language (NLU) is only starting to
emerge. (This e v e n h o l d s for w o r k on any k i n d of p a r a l l e l AI (see e.g. K o w a l i k 1988)). In general, there seem to be two kinds of a p p r o a c h e s to p a r a l l e l NLU. On t h e one hand, there is what we call fine-grain parallelism; on t h e o t h e r hand, t h e r e is coarse-grain parallelism. With fine-grain parallel NLU we r e f e r b a s i c a l l y to the connectionist approach and its decendants. Connectionist models feature h u g e n e t w o r k s of s m a l l n o d e s of information; c o m p u t a t i o n is r e p r e s e n t e d by f l u c t u a t i o n s of the activation levels of nodes and by (parallel) transmission of excitation and inhibition along connections. (For connectionism in general, see Feldman & Ballard 1982, VanLehn 1984, Hillis 1986, McClelland & Rumelhart 1986; for c o n n e c t i o n i s t m o d e l s of NLU, see Cottrell & S m a l l 1983, Cottrell 1985, Pollack & Waltz 1985, M c C l e l l a n d & ~ m e l h a r t 1986) o With coarse-grain parallel N L ~ we r e f e r to a m o r e m o d e s t kind, in w h i c h the s m a l l e s t item
147
of i n f o r m a t i o n is m o r e c o m p l e x tl~an a n o d e in a c o n n e c t i o n l s t m o d e l (it m a y b e a rule, for i n s t a n c e ) ~ b u t in w h i c h one a t t e m p t s to keep the parallel, c o m p u t a t i o n i n v o l v i n g the items of i n f o r m a t i o n m e r e u n d e r c o n t r o l t h a n can be d o n e in a c o n n e c t i o n i s t . m o d e l . (For e x a m p l e s of c o a r s e - g r a i n parallel NLU, see H i r a k a w a 1983 or M a t s u m o t o 3.987). The r e s e a r c h we p r e s e n t h e r e is of the l a t t e r type of p a r a l l e l NI.U. A potentially p a r a l l e l N L U s y s t e m (the W o r d E x p e r t Parser, S m a l l 1980) h a s been drastically r e v i s e d so as to a l l o w a truly parallel implementation (viz. in F l a t C o n C u r r e n t Prolog, using the L o g i x e n v i r o n m e n t (Silverman et al. 1986)); we call the resulting system the P a r a l l e l E x p e r t P a r s e r (PEP, D e v o s 1987).
(WEP)
The Word Expert Parser (WEP, Small 1980) i s a natural language understanding program in the AI tradition of semantic p a r s i n g (see also Hirst 1983v Hahn 1986, Cottrell 1985, Adriaens 1986a/b for WEP-inspired or -related work). The o r g a n i z a t i o n of the model differs strongly from that of a "classical" NLU system. R a t h e r t h a n h a v i n g a n u m b e r of c o m p o n e n t s of rules that are applied (serially) to l i n g u i s t i c input by a g e n e r a l process, W E P considers the words themselves as active a g e n t s (word experts) t h a t i n t e r a c t w i t h e a c h other and with other knowledge s o u r c e s in o r d e r to find the m e a n i n g of a fraQT~ent of text. W o r d s are i m p l e m e n t e d as coroutines~ i.e. processes that run for a while (broadcasting information or performing side-effect operations to refine the representation of the meaning of a text fra~nent), a n d s u s p e n d w h e n t h e y h a v e to w a i t for i n f o r m a t i o n from other experts. The i n f o r m a t i o n t h e y s e n d or w a i t for a r e either s i g n a l s r e l a t i n g to t h e status of t h e p a r s i n g
process (broadcast on a dedicated signal channel) or concepts .that represent the m e a n i n g of parts of the linguistic input (broadcast on a dedicated concept channel). The experts coordinate the understanding process in turn, eventually converging towards a conceptual structure that r e p r e s e n t s t h e m e a n i n g of a t e x t fragment,
30 Fz'Om ~
to PEP
In general, the idea of i n t e r a c t i n g p r o c e s s e s is a very attractive one if one w a n t s a f l e x i b l e p a r s e r c a p a b l e of u s i n g any type of information at a n y m o m e n t it needs it. This basic principle of WEP has b e e n r e t a i n e d [or PEP. Yet, a l t h o u g h t h e design of the sy:3tem s e e m e d to l e n d i t s e l f e a s i l y to a parallel implementation, linguistic and c o m p u t a t i o n a l flaws in the model have made drastic revisions necessary before this could a c t u a l l y ])e done.
3.1 True ]~arallelism
A l t h o u g h WEP c l a i m e d to be " p o t e n t i a l l y p a r a l l e l " , it h e a v i l y (and implicitly) r e l i e d on s e q u e n t i a l i t y to m a k e its p r i n c i p l e s work. E s p e c i a l l y for the restarting of s u s p e n d e d experts, a l a s t - i n first-out regime (stack) t o o k care of contention for m e s s a g e s : the expert that placed an expectation for a m e s s a g e 16.st~ m o s t l y got it first. Also, to a v o i d c o m ~ l i c a t i o n s in e x p e r t communication, no n e w e~perts were i n i t i a l i z e d b e f o r e the q u e u e of l e a d y - t o - r u n e x p e r t s was empty. The a d h e r e n c e to t h i s s e q u e n t i a l i z a t i o n , not to m e n t i o n the s i d e - e f f e c t s involved, obviously made WEP's claim of being "potentially p a r a l l e l " invalid. Ir~ a truly parallel environment, s e q u e n t i a l i t y can no longer be r e l i e d on. PEP uses [parallelism w h e n e v e r p o s s i b l e : for the execution of expert code AND for i n i t i a l i z i n g new e x p e r t s ( i n i t i a l i z i n g all of them as soon as they are read and morphologically analyzed). In o r d e r to r e a l i z e this, the most important departure f r o m the o r i g i n a l m o d e l is t h a t e x p e r t s are no l o n g e r only associated with w o r d s (the only linguistic entities acknowledged by WEP) ° We will now discuss what experts are a s s o c i a t e d with, and how the new v i e w of experts leads to clearer and more explicit c o n c e p t s o~ waiting and c o m m u n i c a t i n g in a parallel environment.
3.2 Wo~d-o:~pQztJ v o z s u s c o n c o p t - e ~ 8 on d i f f e r e n t l e v e l s
A major item of c r i t i c i s m u t t e r e d against WEP h a s b e e n t h a t it c o n s i d e r s the w o r d as the o n l y e n t i t y to b e t u r n e d into an e x p e r t process. Linguistically speaking, the existence of larger constituents is undeniable and must be taken into account, whatever ]aodel o n e advocates. From the
computational viewpoint, squeezing all interactions into words makes it almost i m p o s s i b l e to f i g u r e out w h a t is g o i n g on in the o v e r a l l parsing process. W o r d s h a v e to decide on everything, from morphological issues to pra~natic issues, with jammed c o m m u n i c a t i o n c h a n n e l s as a result. In PEP, experts are associated with c o n c e p t s r a t h e r t h a n w i t h words, it is v e r y natural to do so: words are o n l y u s e d to evoke the concepts that conshitute the m e a n i n g of a fra~lent of text. St:i.li ~ concepts have a c o n c r e t e link to w o r d s and can be r e g a r d e d as b e i n g a s s o c i a t e d w i t h hhe g r o u p of w o r d s that e v o k e s them. E.g. in "the young girl" three concepts can be discovered, associated with the b~sic w o r d - g r o u p s "the", "young" a n d "girl ~'. At: a higher level a compound concept constituting the m e a n i n g of the entire construc~ "the y o u n g g l r l " is invoked. Concretely, in PEP a specific data s t r u c t u r e (the e x p e r t ~rame) is associ~ted with every expert. The hierarchy that o r i g i n a t e s f r o m the c o n c e p t s is r e f l e c t e d by the i n t e r c o n n e c t i o n of the expert fraracs. These are vertically related by ].eve]. interdependencies, and horizonta]ly by the r e l a t i v e role the c o n c e p t s of the frames p l a y in the f r a m e that is b e i n g b u i l t out of them one level higher. Besides its level~ a~ expert frame has three a t t r i b u t e slots: a f u n c t i o n a t t r i b u t e (stating what the role is the expert concept plays at a spec.i.fJc level), a concept attribute (representing the c o n t e n t s of the expert) and a lexical a t t r i b u t e (simply c o r r e s p o n d i n ~ to the gr of words associated with Lhe conc
