Toward relaxing assumptions in languages and the1r implementations

L a n g u a g e i m p l e m e n t o r s f r e q u e n t l y make p r e e m p t i v e dec i s i ons c o n c e r n i n g t h e e x a c t i m p l e m e n t a t i o n s of l a nguage f e a t u r e s . These dec i s i ons cons t r a i n p r o g r a m m e r s ' c o n t r o l o v e r t h e i r c o m p u t a t i o n s and may tempt them to w r i t e i nvo lu ted code to ob t a i n s p e c i a l (o r e f f i c i e n t ) e f f e c t s . In m a n y c a s e s , w e c an d i s t i ngu i sh some p r ope r t i e s of a l anguage f a c i l i t y tha t a r e e s s e n t i a l to t he s e m a n t i c s and o t he r p r o p e r t i e s that are i nc iden ta l . Recen t a b s t r a c t i o n t e c h n i q u e s e m p h a s i z e d e a l i n g w i t h s u ch d i s t i nc t i ons b y s epa r a t i n g the p r o p e r t i e s tha t a r e n e c e s s a r y to p r e s e r v e the seman t i c s f r om the deta i l s for wh i c h some de c i s i o n must b e m a d e b u t m a n y c h o i c e s a r e a dequa t e . We sugges t he r e that t hese ab s t r a c t i o n t e c h n i q u e s c a n b e a p p l i e d to t he p r o b l e m of p r e e m p t i v e language dec i s i ons b y s p e c i f y i n g t he e s s e n t i a l p r o p e r t i e s of l a n g u a g e s fac i l i t i e s in a s k e l e t on base language and de f i n i ng i n t e r f a c e s tha t w i l l a c c e p t a v a r i e t y of imp l emen t a t i o n s that d i f f e r in o the r de ta i l s . T h i s r e s e a r c h w a s s p o n s o r e d b y the Nat iona l S c i ence Founda t i on unde r G ran t M C S 7 7 0 3 8 8 3 a n d b y t h e D e f e n s e A d v a n c e d R e s e a r c h P r o j e c t s A g e n c y (DOD), A R P A O r d e r No . 3 5 9 7 , m o n i t o r e d b y t h e A i r F o r c e A v i o n i c s L a b o r a t o r y Unde r Con t r a c t F 3 3 6 1 5 7 8 C 1 5 5 1 . T h e v i e w s and c o n c l u s i o n s c on t a i n ed in th is document are those of the a u t h o r s a nd s h o u l d n o t b e i n t e r p r e t e d as r e p r e s e n t i n g the o f f i c ia l po l i c i es , e i t he r e x p r e s s e d o r imp l i e d , o f t h e N a t i o n a l S c i e n c e F o u n d a t i o n , the De f en se A d v a n c e d Re sea r ch P r o j e c t s A g e n c y , o r t h e US G o v e r n m e n t .