Implement till 4.4

misc/evaluator.scm

@@ -134,3 +134,27 @@
 (define (first-operand ops) (car ops))
 (define (rest-operands ops) (cdr ops))
 
+(define (cond? exp) (tagged-list? exp 'cond))
+(define (cond-clauses exp) (cdr exp))
+(define (cond-else-clause? clause)
+  (eq? (cond-predicate clause) 'else))
+(define (cond-predicate clause) (car clause))
+(define (cond-actions clause) (cdr clause))
+(define (cond->if exp)
+  (expand-clauses (cond-clauses exp)))
+
+(define (expand-clauses clauses)
+  (if (null? clauses)
+      'false                          ; no else clause
+      (let ((first (car clauses))
+            (rest (cdr clauses)))
+        (if (cond-else-clause? first)
+            (if (null? rest)
+                (sequence->exp (cond-actions first))
+                (error "ELSE clause isn't last -- COND->IF"
+                       clauses))
+            (make-if (cond-predicate first)
+                     (sequence->exp (cond-actions first))
+                     (expand-clauses rest))))))
+
+'evaluator-loaded