;; Konto mit 100 Mark

(define balance 100)

(define withdraw
  (lambda (amount)
    (if (>= balance amount)
	(begin
	  (set! balance (- balance amount))
	  balance)
	'not-enough-money)))

;; Konto mit lokalem Kontostand

(define withdraw
  (let ((balance 100))
    (lambda (amount)
      (if (>= balance amount)
          (begin
            (set! balance (- balance amount))
            balance)
          'not-enough-money))))

;; Fabrik für Konten

(define make-withdraw
  (lambda (balance)
    (lambda (amount)
      (if (>= balance amount)
          (begin
            (set! balance (- balance amount))
            balance)
          'not-enough-money))))

;; Substitutionsmodell und referentielle Transparenz

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

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

;; Fakultät rein funktional

(define factorial
  (lambda (n)
    (letrec ((loop
              (lambda (fac count)
                (if (> count n)
                    fac
                    (loop (* count fac)
                          (+ 1 count))))))
      (loop 1 1))))

;; Fakultät imperativ

(define factorial
  (lambda (n)
    (let ((fac 1)
          (count 1))
      (letrec ((loop
                (lambda ()
                  (if (> count n)
                      fac
                      (begin
                        (set! fac (* count fac))
                        (set! count (+ 1 count))
                        (loop))))))
        (loop)))))