Show description

No global v a r i a b l e references are allowed, 2. Value parameters may not be changed w i t h i n procedure bodies, 3. No f u n c t i o n or p r o c e d u r e parameters are allowed, 4. , array elements are not allowed as v a r i a b l e arguments, 5. The v a r i a b l e argument list may not contain m u l t i p l e occurrences of the same identifier, 6. No v a r i a b l e a r g u m e n t may appear in the value arguments 7. Procedures in a procedure call, and (and functions) may reference only procedures and functions that have been p r e v i o u s l y d e c l a r e d in the p r o g r a m text.

This use of goto seems to be a fairly complex c o n s t r u c t that does not belong in a "subset" like the one being defined here. 6 and Chapter 6, however, we will discuss the problems involved in a l l o w i n g "escapes" out of functions and procedures. The limitations on the use of simple jumps is again p r i m a r i l y one of convenience. It is possible to give both axiomatic and Hoare 1973, Knuth 1974] and m a t h e m a t i c a l [see Clint [see Scott and S t r a c h e y 1972] d e f i n i t i o n s of blocks using a r b i t r a r i l y c o m p l e x sequences of simple jumps.

However, we believe that for readers u n f a m i l i a r w i t h m a t h e m a t i c a l semantics, this informal i n t e r p r e t a t i o n may make the d e f i n i t i o n more r e a d a b l e and understandable. One final point needs to be made about the d e f i n i t i o n to follow. The s e m a n t i c equations given b e l o w are intended to give a m a x i m a l semantics for the P A S C A L subset in the following sense. A l t h o u g h it is r e c o g n i z e d that any i m p l e m e n t a t i o n of the language m a y impose l i m i t a t i o n s on such things as the size of integers, number of array elements, or length of computation, no such r e s t r i c t i o n s will appear in the semantic d e s c r i p t i o n p r e s e n t e d below.