Because EVSETQ can be used to initialize new top-level variables, it is convenient for DRIVER-LOOP-1 to call EVSETQ when defining a new function (see Figure 12). Unlike the DRIVER-LOOP-1 of Figure 10, this one has no special knowledge about the structure of environments; as before, such knowledge is hidden in environment specialists such as BIND, VALUE, and now EVSETQ. (The value of EVSETQ is not used, but thrown away; we introduce an extra throwaway parameter into the definition of DRIVER-LOOP for this purpose.)
(DEFINE (DRIVER)
(DRIVER-LOOP <THE-PRIMITIVE-PROCEOURES>
NIL
(PRINT '|LITHP ITH LITHTENING|)))
(DEFINE (DRIVER-LOOP ENV HUNOZ HUKAIRZ)
(DRIVER-LOOP-1 ENV (READ)))
(DEFINE (DRIVER-LOOP-1 ENV FORM)
(COND ((ATOM FORM)
(DRIVER-LOOP ENV NIL (PRINT (EVAL FORM ENV))))
((EQ (CAR FORM) 'DEFINE)
(DRIVER-LOOP ENV
(EVSETQ (CAADR FORM)
(LIST '&LABELED
(CDADR FORM)
(CADDR FORM))
ENV)
(PRINT (CAADR FORM))))
(T (DRIVER-LOOP ENV NIL (PRINT (EVAL FORM ENV))))))
For EVSETQ see Figure 11.
Figure 12
Driver Loop for Evaluator with User Side Effects
(Assignment to Variables)
(Once we have side effects, we don't really need the &LABELED device to permit incremental definition of recursive functions; we can just perform a side effect on the top-level environment. We left the &LABELED device in Figure 12 for continuity with the previous examples. "Real" LISP systems use the side effect method. See {Note Driver Loop with Side Effects}, and also (Note LABELS with Side Effects).)
