counters.scm

(define (make-counter-1)
  (let ((counter 0))
    (lambda ()      
      (set! counter (+ counter 1))
      counter)))

(define (make-counter-2)
  (define counter 0)
    (lambda ()      
      (set! counter (+ counter 1))
      counter))

(define (make-counter-3)
  (define counter 0)
  (define (count)
    (set! counter (+ counter 1))
    counter)
  count)
  
;; in the REPL ...
  
> (define counter-1 (make-counter-1))
> (define counter-2 (make-counter-2))
> (define counter-3 (make-counter-2))

> (counter-1)
1
> (counter-2)
1
> (counter-3)
1
> (counter-1)
2
> (counter-2)
2
> (counter-3)
2
> (counter-1)
3
> (counter-1)
4
> (counter-1)
5
> (counter-2)
3
> (counter-2)
4
> (counter-2)
5
> (counter-3)
3
> (counter-3)
4
> (counter-3)
5