General Computer Science 320201 GenCS I & II Lecture ... - Kwarc

A SW program (see the next slide for an example) first declares a set of variables (type declaration),
executes a statement (type sta), and finally returns an expression (type exp). Expressions of SW
can read the values of variables, but cannot change them. The statements of SW can read and
change the values of variables, but do not return values (as usual in imperative languages). Note
that SW follows common practice in imperative languages and models the conditional as a statement.
Concrete vs. Abstract Syntax of a SW Program
Example 469 (Abstract SW Syntax) We apply the abstract syntax to the SW program
from Example 467:
var n:= 12; var a:= 1;
while 2 wln(x)+y) 0 xs
fun wln(con _)=2 | wln(add)=1 | wln(sub)=1 | wln(mul)=1 | wln(leq)=1
| wln(jp _)=2 | wln(cjp _)=2
| wln(peek _)=2 | wln(poke _)=2 | wln(halt)=1
c○: Michael Kohlhase 299
Before we can come to the implementation of the compiler, we will need an infrastructure for
environments.
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