练习给出的过程定义：
;;; 10-make-withdraw.scm
(define (make-withdraw initial-amount)
(let ((balance initial-amount))
(lambda (amount)
(if (>= balance amount)
(begin (set! balance (- balance amount))
balance)
"Insufficient funds"))))
可以转换成以下 lambda 定义：
;;; 10-make-withdraw-using-lambda.scm
(define make-withdraw
(lambda (initial-amount)
((lambda (balance)
(lambda (amount)
(if (>= balance amount)
(begin (set! balance (- balance amount))
balance)
"Insufficient funds")))
initial-amount)))
这个 lambda 版本的 make-withdraw 创建以下过程对象：
+------------------------------------+
global env -> | |
| make-withdraw --+ |
+-------------------|----------------+
| ^
| |
v |
[*][*]----+
|
|
v
parameters: initial-amount
body: ((lambda (balance)
(lambda (amount)
(if (>= balance amount)
(begin (set! balance (- balance amount))
balance)
"Insufficient funds")))
initial-amount)
如果执行表达式 (define w1 (make-withdraw 100)) ，我们得到以下环境：
+------------------------------------+
global env -> | |
| |
+------------------------------------+
^
(make-withdraw 100) |
|
+--------------+
| |
E1 -> | initial: 100 |
| |
+--------------+
((lambda (balance)
(lambda (amount)
(if (>= balance amount)
(begin (set! balance (- balance amount))
balance)
"Insufficient funds")))
initial)
因为 make-withdraw 的函数体内又是一个函数调用，所以以上的求值又引发以下的求值发生。
首先是创建又一个过程对象：
+------------------------------------+
global env -> | |
| |
+------------------------------------+
^
(make-withdraw 100) |
|
+--------------+
| |
E1 -> | initial: 100 |
| |
+--------------+
| ^
| |
| |
[*][*]-------+
|
|
v
parameters: balance
body: (lambda (amount)
(if (>= balance amount)
(begin (set! balance (- balance amount))
balance)
"Insufficient funds"))
而这个新的过程对象会即刻被调用，继而产生又一个新环境：
+------------------------------------+
global env -> | |
| |
+------------------------------------+
^
(make-withdraw 100) |
|
+--------------+
| |
E1 -> | initial: 100 |
| |
+--------------+
^
((lambda (balance) ...) 100) |
|
+--------------+
| |
E2 -> | balance: 100 |
| |
+--------------+
(lambda (amount)
(if (>= balance amount)
(begin (set! balance (- balance amount))
balance)
"Insufficient funds"))
(lambda (balance) ...) 的体内是另一个 lambda 表达式 (lambda (amount) ...) ，因此我们要为它创建又一个过程对象：
+------------------------------------+
global env -> | |
| |
+------------------------------------+
^
(make-withdraw 100) |
|
+--------------+
| |
E1 -> | initial: 100 |
| |
+--------------+
^
((lambda (balance) ...) 100) |
|
+--------------+
| |
E2 -> | balance: 100 |
| |
+--------------+
| ^
| |
v |
[*][*]-----+
|
|
v
parameters: amount
body: (if (>= balance amount)
(begin (set! balance (- balance amount))
balance)
"Insufficient funds")
对 (make-withdraw 100) 的求值过程到此就暂告一段落了，这时，可以将符号 w1 和所得的过程对象建立约束(bundle)了：
+-------------------------------------------+
global env -> | |
| w1 |
+---|---------------------------------------+
| ^
| (make-withdraw 100) |
| |
| +--------------+
| | |
| E1 -> | initial: 100 |
| | |
| +--------------+
| ^
| ((lambda (balance) ...) 100) |
| |
| +--------------+
| | |
| E2 -> | balance: 100 |
| | |
| +--------------+
| | ^
| | |
| v |
+------------------> [*][*]-----+
|
|
v
parameters: amount
body: (if (>= balance amount)
(begin (set! balance (- balance amount))
balance)
"Insufficient funds")
使用之前求值得到的 w1 ，执行表达式 (w1 50) ，会创建以下环境：
+-------------------------------------------+
global env -> | |
| w1 |
+---|---------------------------------------+
| ^
| (make-withdraw 100) |
| |
| +--------------+
| | |
| E1 -> | initial: 100 |
| | |
| +--------------+
| ^
| ((lambda (balance) ...) 100) |
| |
| +--------------+
| | |
| E2 -> | balance: 100 |
| | |
| +--------------+
| | ^ ^
| | | | +------------+
| v | | | |
+------------------> [*][*]-----+ +------------------------------------| amount: 50 | <- E3
| | |
| +------------+
v
parameters: amount (if (>= balance amount)
body: (if (>= balance amount) (begin (set! balance (- balance amount))
(begin (set! balance (- balance amount)) balance)
balance) "Insufficient funds")
"Insufficient funds")
环境 E3 在执行过程体中的表达式之后消失， E2 的 balance 被设置为 50 ，以下是求值完毕之后的环境图：
+-------------------------------------------+
global env -> | |
| w1 |
+---|---------------------------------------+
| ^
| (make-withdraw 100) |
| |
| +--------------+
| | |
| E1 -> | initial: 100 |
| | |
| +--------------+
| ^
| ((lambda (balance) ...) 100) |
| |
| +--------------+
| | |
| E2 -> | balance: 50 |
| | |
| +--------------+
| | ^
| | |
| v |
+------------------> [*][*]-----+
|
|
v
parameters: amount
body: (if (>= balance amount)
(begin (set! balance (- balance amount))
balance)
"Insufficient funds")
最后，定义另一个 make-withdraw 实例 w2 ，它的功能性和 w1 类似，但是却保存着自己的一簇状态变量和过程对象（最明显的就是它们各自的 balance 变量）：
+-----------------------------------------------------------------------------------------+
global env -> | |
| w1 w2 |
+---|-----------------------------------------|-------------------------------------------+
| ^ | ^
| (make-withdraw 100) | | |
| | | |
| +--------------+ | +--------------+
| | | | | |
| E1 -> | initial: 100 | | E1 -> | initial: 100 |
| | | | | |
| +--------------+ | +--------------+
| ^ | ^
| ((lambda (balance) ...) 100) | | ((lambda (balance) ...) 100) |
| | | |
| +--------------+ | +--------------+
| | | | | |
| E2 -> | balance: 50 | | E2 -> | balance: 100 |
| | | | | |
| +--------------+ | +--------------+
| | ^ | | ^
| | | | | |
| v | | v |
+------------------> [*][*]-----+ +----------------------->[*][*]----+
| |
| |
v v
parameters: amount parameters: amount
body: (if (>= balance amount) body: (if (>= balance amount)
(begin (set! balance (begin (set! balance
(- balance amount)) (- balance amount))
balance) balance)
"Insufficient funds") "Insufficient funds")