练习3-45

原文

Exercise 3.45. Louis Reasoner thinks our bank-account system is unnecessarily complex and error-prone now that deposits and withdrawals aren’t automatically serialized. He suggests that make-accountand-serializer should have exported the serializer (for use by such procedures as serializedexchange) in addition to (rather than instead of) using it to serialize accounts and deposits as makeaccount did. He proposes to redefine accounts as follows:

 (define (make-account-and-serializer balance) 
    (define (withdraw amount)   
       (if (>= balance amount)     
           (begin (set! balance (- balance amount))                
                   balance)       
            "Insufficient funds")) 
    (define (deposit amount)  
       (set! balance (+ balance amount))
       balance) 
    (let ((balance-serializer (make-serializer)))   
       (define (dispatch m)  
          (cond ((eq? m ‘withdraw) (balance-serializer withdraw))            
                ((eq? m ‘deposit) (balance-serializer deposit))            
                ((eq? m ‘balance) balance)    
                ((eq? m ‘serializer) balance-serializer)            
                (else (error "Unknown request -- MAKE-ACCOUNT"                         
                             m))))  
        dispatch))

Then deposits are handled as with the original make-account:

(define (deposit account amount) 
   ((account ‘deposit) amount))

Explain what is wrong with Louis’s reasoning. In particular, consider what happens when serializedexchange is called.

代码

待完成