3.1.4.6 Example 1: A Serial Number Generator

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

When make-serial-number-generator is called, it creates a local environment with a binding for current-serial-number whose 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 the current-serial-number binding in the captured environment and then returns the current value.

Note that 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 make-serial-number-generator creates a new local let environment and returns a new procedure object with an association to this environment.