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...