GNU Info

(guile.info)Serial Number

---

Next: Shared Variable Prev: Closure Up: About Closure

---

Enter node , (file) or (file)node

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.