balance.rkt

#lang racket

;;; rozdział 3.1.1

(define balance 100)

(define (withdraw amount)
  (if (>= balance amount)
      (begin (set! balance (- balance amount))
             balance)
      "Insufficient funds"))

;: (withdraw 25)
;: (withdraw 25)
;: (withdraw 60)
;: (withdraw 15)

(define new-withdraw
  (let ((balance 100))
    (lambda (amount)
      (if (>= balance amount)
          (begin (set! balance (- balance amount))
                 balance)
          "Insufficient funds"))))


(define (make-withdraw balance)
  (lambda (amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds")))


;: (define W1 (make-withdraw 100))
;: (define W2 (make-withdraw 100))
;: (W1 50)
;: (W2 70)
;: (W2 40)
;: (W1 40)

(define (make-account balance password)
  (define (withdraw amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds"))
  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)
  (define (incorrect-pass i)
    'incorect-pass)
  (define (change-pass p)
    (set! password p))
  (define (dispatch m p)
    (cond ((not (eq? p password)) incorrect-pass)
          ((eq? m 'change-pass) change-pass)
          ((eq? m 'withdraw) withdraw)
          ((eq? m 'deposit) deposit)
          (else (error "Unknown request -- MAKE-ACCOUNT"
                       m))))
  dispatch)

;: (define acc (make-account 100))

;: ((acc 'withdraw) 50)
;: ((acc 'withdraw) 60)
;: ((acc 'deposit) 40)
;: ((acc 'withdraw) 60)

;;; rozdział 3.1.3

(define (make-simplified-withdraw balance)
  (lambda (amount)
    (set! balance (- balance amount))
    balance))


;: (define W (make-simplified-withdraw 25))
;: (W 20)
;: (W 10)


(define (make-decrementer balance)
  (lambda (amount)
    (- balance amount)))

;: (define D (make-decrementer 25))
;: (D 20)
;: (D 10)

;: ((make-decrementer 25) 20)
;: ((lambda (amount) (- 25 amount)) 20)
;: (- 25 20)

;: ((make-simplified-withdraw 25) 20)

;: ((lambda (amount) (set! balance (- 25 amount)) 25) 20)
;: (set! balance (- 25 20)) 25

;;; Toższamość obiektów

;: (define D1 (make-decrementer 25))
;: (define D2 (make-decrementer 25))
;:
;: (define W1 (make-simplified-withdraw 25))
;: (define W2 (make-simplified-withdraw 25))
;:
;: (W1 20)
;: (W1 20)
;: (W2 20)

;: (define peter-acc (make-account 100))
;: (define paul-acc (make-account 100))
;:
;: (define peter-acc (make-account 100))
;: (define paul-acc peter-acc)

;;; Kłopoty z programowaniem imperatywnym

#|(define (factorial n)
  (define (iter product counter)
    (if (> counter n)
        product
        (iter (* counter product)
              (+ counter 1))))
  (iter 1 1))
|#
(define (factorial n)
  (let ((product 1)
        (counter 1))
    (define (iter)
      (if (> counter n)
          product
          (begin (set! product (* counter product))
                 (set! counter (+ counter 1))
                 (iter))))
    (iter)))

(define (cons x y)
  (define (set-x! v) (set! x v))
  (define (set-y! v) (set! y v))
  (define (dispatch m)
    (cond ((eq? m 'car) x)
          ((eq? m 'cdr) y)
          ((eq? m 'set-car!) set-x!)
          ((eq? m 'set-cdr!) set-y!)
          (else (error "Undefined operation -- CONS" m))))
  dispatch)

(define (car z) (z 'car))
(define (cdr z) (z 'cdr))

(define (set-car! z new-value)
  ((z 'set-car!) new-value)
  z)

(define (set-cdr! z new-value)
  ((z 'set-cdr!) new-value)
  z)
;; zad 2
(define (make-cycle l)
  (define (aux x)
    (if (null? (mcdr x))
        (set-mcdr! x l)
        (aux (mcdr l))))
  (aux l))