Rewrite require as a special form (Exercise 4.54)

Tested this with the "liars" problem, and the result is the same.

mceval/amb-utilities.rkt

@@ -6,10 +6,7 @@
 
 (define amb-utilities-program
 
-  '((define (require p)
-      (if (not p) (amb)))
-    
-    (define (an-element-of items)
+  '((define (an-element-of items)
       (require (not (null? items)))
       (amb (car items) (an-element-of (cdr items))))
     

mceval/ambeval.rkt

@@ -21,6 +21,9 @@
 (define (amb-form exp) (car exp))       ; amb or ramb
 (define (amb-choices exp) (cdr exp))
 
+(define (require? exp) (tagged-list? exp 'require))
+(define (require-predicate exp) (cadr exp))
+
 ;; analyze from 4.1.6, with clause from 4.3.3 added
 ;; and also support for Let
 (define (analyze exp)
@@ -39,6 +42,7 @@
         ((let? exp) (analyze (let->combination exp))) ;**
         ((amb? exp) (analyze-amb exp))                ;**
         ((ramb? exp) (analyze-amb exp))
+        ((require? exp) (analyze-require exp))
         ((application? exp) (analyze-application exp))
         (else
          (error "Unknown expression type -- ANALYZE" exp))))
@@ -229,6 +233,17 @@
                                        (try-next (rest-choices choices)))))))
       (try-next cprocs))))
 
+(define (analyze-require exp)
+  (let ((pproc (analyze (require-predicate exp))))
+    (lambda (env succeed fail)
+      (pproc env
+             (lambda (pred-value fail2)
+               (if (false? pred-value)
+                   (fail)
+                   (succeed 'ok fail2)))
+             fail))))
+
+
 ;; if-fail is used to catch failure of an expression
 (define (analyze-if-fail exp)
   (let ((tproc (analyze (if-fail-test exp)))