#lang racket

; create some functions to track how often something
; is used.  It modifies a global state variable.

(define global-eval-count 0)
(define (reset-count) (set! global-eval-count 0))
(define (get-count) global-eval-count)
(define (inc-count)
  (set! global-eval-count (+ 1 global-eval-count))
  (displayln (format "eval count ~a" global-eval-count))
  )

; define syntax for weak delay of expr.  Returns a 
; closure that when evaluated evaluates expr in the 
; original context.
(define-syntax-rule (w-delay expr) 
  (lambda ()
    (inc-count)  ; for tracing purposes only
    expr
    ))

; define syntax for weak force of a delayed expr
; simply evaluates the lambda
(define-syntax-rule (w-force thunk)
  (thunk)
  )

(define (test-it)
  (reset-count)
  (define v1 (w-delay (+ 40 2)))
  (displayln (w-force v1))
  (displayln (w-force v1))
  
  
  ; f1 assumes that w-y is a delayed expression
  (define (f1 n w-y)
    (if (= n 0) 1 (+ (w-force w-y) 1)))
  
  (define x 0)
  (display "on 0: ") (displayln (f1 0 (w-delay (/ 1 x))))
  
  (display "on 1: ") (displayln (f1 1 (w-delay (/ 1 x))))
  )