Toward relaxing assumptions in languages and their implementations

L a n g u a g e implementors frequently make pre-emptive decisions concerning the exact i m p l e m e n t a t i o n s of language features. These decisions constrain programmers' c o n t r o l over 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 involuted code to obtain special (or e f f i c i e n t ) e f f e c t s . In many cases, we can distinguish some properties of a language f a c i l i t y that are e s s e n t i a l to the semantics and other properties that are incidental. Recent a b s t r a c t i on t e c h n i q u e s emphasize dealing with such distinctions by separating the p r o p e r t i e s that are n e c e s s a r y to p r e s e r v e the semantics from the details for which some decision must be made but many c h o i c e s are adequate. We suggest here that these abstraction techniques can be a p p l i e d to the problem of pre-emptive language decisions by specifying the essential p r o p e r t i e s of languages f a c i l i t i e s in a skeleton base language and defining i n t e r f a c e s that will a c c e p t a v a r i e t y of implementations that differ in other details.