sicp-3-4-1-the-nature-of-time-in-concurrent-systems

#lang racket

;;;;;;;;;;
;; 3.38 ;;
;;;;;;;;;;

(define (make-account initial-balance)
  (define balance initial-balance)
  (define (deposit amount) (set! balance (+ balance amount)))
  (define (withdraw amount) (set! balance (- balance amount)))
  (define (halve) (set! balance (/ balance 2)))
  (define (dispatch m)
    (cond [(eq? m 'deposit) deposit]
          [(eq? m 'withdraw) withdraw]
          [(eq? m 'halve) halve]
          [(eq? m 'balance) balance]
          [else (error "Unknown message -- DISPATCH" m)]))
  dispatch)

(define (peter acct) ((acct 'deposit) 10))
(define (paul acct) ((acct 'withdraw) 20))
(define (mary acct) ((acct 'halve)))

;; What are the possible final balances after allowing these three transactions to
;; be completed sequentially in any order?

(define (test sequence)
  (define test-account (make-account 100))
  (for ([p sequence]) (p test-account))
  (test-account 'balance))

(for/list ([seq (in-permutations (list peter paul mary))])
          (test seq))
;; '(40 50 40 35 45 45)

;; What are some other values that could be produced if the processes could be
;; interleaved?

;; Interleaving the processes allows us to ignore one or two commands because
;; those commands could always be executed between the third command's read and
;; write operations.  So one new value would be 110, obtained by the following
;; sequence of events:

;; peter reads balance = 100
;; peter computes (+ balance 10) and gets 110
;; paul does his transaction
;; mary does her transaction
;; peter sets balance to 110