sicp exercise 3.39

;; Exercise 3.39.  Which of the five possibilities in the parallel execution shown above remain if we instead serialize execution as follows:
;;
;; (define x 10)
;;
;; (define s (make-serializer))
;;
;; (parallel-execute (lambda () (set! x ((s (lambda () (* x x))))))
;;                   (s (lambda () (set! x (+ x 1)))))

;; Required for (parallel) on Guile
(use-modules (ice-9 threads))

;; Following is the version of (make-serializer) on Guile
(define (make-serializer)
  (let ((mutex (make-mutex)))
    (lambda (p)
      (define (serialized-p . args)
        (lock-mutex mutex)
        (let ((val (apply p args)))
          (unlock-mutex mutex)
          val))
      serialized-p)))

(define x 10)

(define s (make-serializer))

(parallel ((lambda () (set! x ((s (lambda () (* x x)))))))
          ((s (lambda () (set! x (+ x 1))))))
(display x) (newline)
;; Evaluates to 101
;; But if the order is swapped, the output is changed

(set! x 10)
(parallel  ((s (lambda () (set! x (+ x 1)))))
           ((lambda () (set! x ((s (lambda () (* x x))))))))

(display x) (newline)
;; Evaluates to 121

;; Ans:
;; theoretical possibiities are : 121, 100, 101