An Overview of a Procedural Approach to Semantic Networks

Apar t from the conceptual" gains o f f e r ed by having a taxonomy of c l a s s e s , the main purpose of the ISA h i e r a r c h y is the i n h e r i t a n c e o f d e f i n i t i o n a l p r o p e r t i e s which is a g e n e r a l i z a t i o n of the d e f a u l t mechanism t h a t c a p i t a l i z e s on the expected s i m i l a r i t y between a subclass and a superc l a s s . The PARTOF h i e r a r c h y , on the o ther hand, i nvo l ves the compos i t ion of groups o f ob jec ts i n t o f u n c t i o n a l u n i t s . Such u n i t s are c a l l e d s t r u c t u r e s and the ob jec ts t ha t c o n s t i t u t e them, t h e i r p a r t s . For c l asses , the p a r t s are v a r i a b l e s ( s l o t s ) which are bound to ob jec t s ( s l o t f i l l e r s ) whenever the c lass i s i n s t a n t i a t e d The s l o t s may have d e f a u l t va lues and r e l a t i o n s c a l l e d dependencies a t t a c h ing them to o ther ob jec ts and hav ing many poss ib l e p rocedura l i n t e r p r e t a t i o n s such as de te rmin ing the a c c e p t a b i l i t y of a s l o t f i l l e r o r i n f e r r i n g i t s va lue . Whereas r e l a t i o n s represent the a s s e r t i o n a l p roper t i e s o f a c l a s s , s l o t s represent the d e f i n i t i o n a l p r o p e r t i e s . Consequent ly , the PARTOF h ie ra r chy determines the i nhe r i t a b l e p r o p e r t i e s of a c lass w h i l e the ISA h ie ra r chy s p e c i f i e s the i n h e r i t a n c e pa ths . The metaclass "program" ( the c lassof a l l programs) can be thought of as as t r u c t u r e having p r e r e q u i s i t e , e f f e c t e t c .as s l o t s and each program f i l l s theses l o t s by p r o v i d i n g p a r t i c u l a r p r e r e q u i s i t e se t c . This a l lows programs to be i n t e g r a t e di n t o the two h i e r a r c h i e s and b e n e f i t f romthe o r g a n i z a t i o n ( e . g . i n h e r i t a n c e ) l i k eany o ther c l a s s . S i m i l a r l y , the metaclass" c l a s s " is a s t r u c t u r e whose s l o t s rep re -sent the fou r bas ic programs f o r c lassesand the semantics of each c lass are de f i nedby f i l l i n g these s l o t s by s p e c i f i c programs.The d e f a u l t va lues f o r these s l o t s (andt h e : f o u r s i m i l a r ones i n " r e l a t i o n " )c o n s t i t u t e the s tandard behav iour o fc lasses and r e l a t i o n s p r o v i d i n g the p r i m i -t i v e s in terms o f which e v e r y t h i n g e lsecan be de f i ned and l i n k i n g the fo rma l i smto a p h y s i c a l imp lementa t ion . References[Levesque 77] Levesque, H. "A p rocedu ra lapproach to semantic n e t w o r k s " , TR-105,Dept. of Computer Sc ience, U. of To ron to ,1977. [Hayes 74] Hayes, P .J . "Some pro -blems and non-problems in r e p r e s e n t a t i o nt h e o r y " , Proc. AISB summer conference,1974.[ A b r i a l 74] A b r i a l , J .R. "Data semant ics"in Data Management Systems, K l imb ie and Koffeman ( e d s . ) , N o r t h H o l l a n d , 1974.