Practice 4.2

a

修改后的程序

	(define (eval exp env)
	  (cond ((self-evaluating? exp) exp)
	        ((variable? exp) (lookup-variable-value exp env))
	        ((quoted? exp) (text-of-quotation exp))
	        ((application? exp)
	         (apply (eval (operator exp) env)
	                (list-of-values (operands exp) env)))
	        ((assignment? exp) (eval-assignment exp env))
	        ((definition? exp) (eval-definition exp env))
	        ((if? exp) (eval-if exp env))
	        ((lambda? exp)
	         (make-procedure (lambda-parameters exp)
	                         (lambda-body exp)
	                         env))
	        ((begin? exp)
	         (eval-sequence (begin-actions exp) env))
	        ((cond? exp) (eval (cond->if exp) env))
	        (else
	          (error "Unknown expression type -- EVAL" exp))))

因为书中application?的定义是不属于它之前的各种表达式类型的任意复合表达式才满足application?子句,提前它的顺序的话，就不满足书中的定义了。会把所有的非自求值变量和引用表达式,都当成过程比如在求值 (define x 3)的时候,按修改后的顺序直接满足application?子句,会执行下面的过程

	(apply (eval (operator exp) env)
	       (list-of-values (operands exp) env))

operator和operands的定义如下

	(define (operator exp) (car exp))
	(define (operands exp) (cdr exp))

执行如下过程：

	(apply (eval define env)
	       (list-of-values '(x 3) env))

执行define这个过程 ,参数是’(x 3),然而此时在被求值的环境中，没有define的约束，所以会出错

b

只需要修改application?和operator operands的定义

	(define (application? exp)
	  (tagged-list? exp 'call))

	(define (operator exp) (cadr exp))
	(define (operands exp) (cddr exp))