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This example uses closure to create two procedures,
deposit, that both refer to the same captured local
environment so that they can both access the
binding inside that environment. The value of this variable binding
persists between calls to either procedure.
Note that the captured
balance variable binding is private to
these two procedures: it is not directly accessible to any other code.
It can only be accessed indirectly via
deposit, as illustrated by the
(define get-balance #f) (define deposit #f) (let ((balance 0)) (set! get-balance (lambda () balance)) (set! deposit (lambda (amount) (set! balance (+ balance amount)) balance))) (define (withdraw amount) (deposit (- amount))) (get-balance) ⇒ 0 (deposit 50) ⇒ 50 (withdraw 75) ⇒ -25
An important detail here is that the
deposit variables must be set up by
defineing them at top
level and then
set!ing their values inside the
define within the
let body would not work: this
would create variable bindings within the local
that would not be accessible at top level.