#lang racket

; using closures to capture and hide state

; (hide-state) returns a closure that when invoked increments the 
; previous stored value and then returns it.  The initial value is 0.

(define hide-state
  (lambda  () 
    (let ( (internal 0) )  ; internal is not visible outside of hide-state
      (lambda () (set! internal (+ internal 1)) internal)
      )
    )
  )

(display "hide-state")(newline)
(define i (hide-state))
(i)
(i)
(i)

; (make-counter init incr) returns a list of 3 closures.
; the first returns the current value of the counter
; the second adds incr and returns the current value
; the third subtracts incr and returns the current value

(define make-counter
  (lambda (init incr)
    (let ( (counter init) )
      (list 
       (lambda () counter) ; return the current value
       (lambda () (set! counter (+ counter incr)) counter) ; increment and return the current value
       (lambda () (set! counter (- counter incr)) counter) ; decrement and return the current value
       ))))

(newline)
(display "make-counter")(newline)
(define counter (make-counter 5 3))
(define v (first counter))
(define v+ (second counter))
(define v- (third counter))

(v)
(v+)
(v)
(v+)
(v-)
(v)
(v-)