This example uses closure to create a procedure with a variable binding that is private to the procedure, like a local variable, but whose value persists between procedure calls.
(define (make-serial-number-generator) (let ((current-serial-number 0)) (lambda () (set! current-serial-number (+ current-serial-number 1)) current-serial-number))) (define entry-sn-generator (make-serial-number-generator)) (entry-sn-generator) ⇒ 1 (entry-sn-generator) ⇒ 2
make-serial-number-generator is called, it creates a local
environment with a binding for
initial value is 0, then, within this environment, creates a procedure.
The local environment is stored within the created procedure object and
so persists for the lifetime of the created procedure.
Every time the created procedure is invoked, it increments the value of
current-serial-number binding in the captured environment and
then returns the current value.
make-serial-number-generator can be called again to
create a second serial number generator that is independent of the
first. Every new invocation of
creates a new local
let environment and returns a new procedure
object with an association to this environment.