In section 3.2.3 we saw how the environment model described the behavior of procedures with local state. Now we have seen how internal definitions work. A typical message-passing procedure contains both of these aspects. Consider the bank account procedure of section 3.1.1 :
(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)
Show the environment structure generated by the sequence of interactions
(define acc (make-account 50)) ((acc 'deposit) 40) 90 ((acc 'withdraw) 60) 30
Where is the local state for
kept? Suppose we define another account
(define acc2 (make-account 100))
How are the local states for the two accounts kept distinct? Which parts of the environment structure are shared between
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