练习4-7

原文

Exercise 4.7. Let* is similar to let, except that the bindings of the let variables are performed sequentially from left to right, and each binding is made in an environment in which all of the preceding bindings are visible. For example

(let* ((x 3)      
       (y (+ x 2))  
       (z (+ x y 5)))  
   (* x z))

returns 39. Explain how a let* expression can be rewritten as a set of nested let expressions, and write a procedure let*->nested-lets that performs this transformation. If we have already implemented let (exercise 4.6) and we want to extend the evaluator to handle let*, is it sufficient to add a clause to eval whose action is

(eval (let*->nested-lets exp) env)

or must we explicitly expand let* in terms of non-derived expressions?

分析

这道题和上一道很类似，抓住题中的要点就会迎刃而解啦。那就是说从左至右求值，那么我们可以用list和car以及cdr来完成，核心思想是用递归，不断的向右边推进，直到exp为空，此时就返回body，然后结束构造。至于tagged-list?这些和上一题都是一样的。

代码

(define (let*? expr) (tagged-list? expr ‘let*))

(define (let*-body expr) (caddr expr))

(define (let*-exp expr) (cadr expr))

(define (let*->nested-lets expr)
  (let ((exp (let*-exp expr))
    (body (let*-body expr)))
    (defien (make-lets exprs)
      (if (null? exprs)
      body
      (list ‘let
        (list (car exprs))
        (make-lets (cdr exprs)))))
    (make-lets exp)))