Exercise 4.18 · 习题

Exercise 4.18: Consider an alternative strategy
for scanning out definitions that translates the example in the text to

(lambda ⟨vars⟩
(let ((u '*unassigned*)
(v '*unassigned*))
(let ((a ⟨e1⟩)
(b ⟨e2⟩))
(set! u a)
(set! v b))
⟨e3⟩))

Here a and b are meant to represent new variable names, created
by the interpreter, that do not appear in the user’s program. Consider the
solve procedure from 3.5.4:

(define (solve f y0 dt)
(define y (integral (delay dy) y0 dt))
(define dy (stream-map f y))
y)

Will this procedure work if internal definitions are scanned out as shown in

this exercise? What if they are scanned out as shown in the text? Explain.

练习 4.18：考虑一种扫描出定义的替代策略，它将正文中的示例变换为

(lambda ⟨vars⟩
(let ((u '*unassigned*)
(v '*unassigned*))
(let ((a ⟨e1⟩)
(b ⟨e2⟩))
(set! u a)
(set! v b))
⟨e3⟩))

这里 a 和 b 表示解释器创建的新变量名，它们不出现在用户程序中。考虑 3.5.4 中的 solve 过程：

(define (solve f y0 dt)
(define y (integral (delay dy) y0 dt))
(define dy (stream-map f y))
y)

如果内部定义按本练习所示方式扫描出，这个过程能正常工作吗？如果按正文中所示方式扫描出呢？请解释。