(guile.info.gz) Serial Number

Info Catalog

(guile.info.gz) Closure

(guile.info.gz) About Closure

(guile.info.gz) Shared Variable

 
 14.4.6 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.
 

Info Catalog

(guile.info.gz) Closure

(guile.info.gz) About Closure

(guile.info.gz) Shared Variable