SICP Exercise 4.7
a)爲什麼let*表達式可以重寫爲一些嵌套的let表達式?
因爲嵌套的let表達式可以讓已經做了的約束都是可見的。例如:
(let* ((x 3)
(y (+ x 2))
(z (+ x y 5)))
(* x z))
可以重寫爲:
(let ((x 3))
(let ((y (+ x 2)))
(let ((z (+ x y 5)))
(* x z))))
b)下面我們參考該例子,寫出把let*表達式轉化爲let表達式的實現:
(define (let*->nested-lets exp)
(define (expand-clauses bindings body)
(if (null? bindings)
body
(make-let (list (car bindings))
(expand-clauses (cdr bindings) body))))
(expand-clauses (cadr exp) (caddr exp)))
(define (eval-let* exp env)
(eval (let*->nested-lets exp) env))
c)可以向eval中添加(eval (let*->nested-lets exp) env)
但是我更喜歡以數據導向的方式,所以只需再增加一條語句:
(put 'let* eval-let*)
;;; M-Eval input:
(let* ((x 3)
(y (+ x 1)))
(+ x y))
;;; M-Eval value:
7