Uses an internal lambda for dispatching on messages.
We can also create objects that handle deposits as well as withdrawals, and thus we can represent simple bank accounts. Here is a procedure that returns a “bank-account object” with a specified initial balance:
(define (make-account balance)
(define (withdraw amount)
(if (>= balance amount)
(begin (set! balance
(- balance amount))
balance)
"Insufficient funds"))
(define (deposit amount)
(set! balance (+ balance amount))
balance)
(define (dispatch m)
(cond ((eq? m 'withdraw) withdraw)
((eq? m 'deposit) deposit)
(else (error "Unknown request:
MAKE-ACCOUNT" m))))
dispatch)
Each call to make-account sets up an environment with a local state variable balance. Within this environment, make-account defines procedures deposit and withdraw that access balance and an additional procedure dispatch that takes a “message” as input and returns one of the two local procedures. The dispatch procedure itself is returned as the value that represents the bank-account object. This is precisely the message-passing style of programming that we saw in 2.4.3, although here we are using it in conjunction with the ability to modify local variables.3.1.1 Local State Variables
https://sarabander.github.io/sicp/html/3_002e1.xhtml#g_t3_00...
