Implement 5.23

misc/sicp-eceval-support.scm

@@ -95,6 +95,49 @@
     (scan (frame-variables frame)
           (frame-values frame))))
 
+;;; cond-support
+(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))))))
+
+;;; lambda-support
+
+;;; let-support
+(define (let? exp) (tagged-list? exp 'let))
+(define (let-bindings exp) (cadr exp))
+(define (let-body exp) (cddr exp))
+(define (let-binding-var binding) (car binding))
+(define (let-binding-exp binding) (cadr binding))
+(define (let-vars exp) (map let-binding-var (let-bindings exp)))
+(define (let-exps exp) (map let-binding-exp (let-bindings exp)))
+
+(define (make-lambda parameters body)
+  (cons 'lambda (cons parameters body)))
+
+(define (let->combination exp)
+  (let ((let-variables (let-vars exp))
+        (let-expressions (let-exps exp)))
+    (cons (make-lambda let-variables (let-body exp))
+          let-expressions)))
 
 ;;;SECTION 4.1.4