Practice 4.26

把unless定义为if的派生表达式

(define (unless? exp) (tagged-list? exp 'unless))

(define (unless-condition exp)
  (cadr exp))

(define (unless-usual-clause exp)
  (caddr exp))

(define (unless-exceptional-clause)
  (cadddr exp))

(define (unless->if exp)
  (let ((predicate (unless-condition exp))
        (consequent (unless-usual-clause exp))
        (alternative (unless-exceptional-clause exp)))
      (make-if predicate consequent alternative)))

在解释器中加入对unless的处理

((unless? exp) (eval (unless->if exp)))

如果是派生表达式，看以下高阶过程中

(define (foo bar a b c)
  (bar a b c))

求值以下过程

(foo unless a b c)

在求值参数unless的时候在环境中找不到unless就会出错

如果unless是过程，在这个过程的求值中就不会出现问题