CS 251, Fall 2024, Carleton College, Dr. Joshua R. Davis



Is Scheme call-by-value? We never asked that until now, because our functions 
didn't mutate their arguments, because we wrote in a functional style. But how 
would we test it?

(define x 3)

(define test
  (lambda (y)
    (set! y 5)
    (display y)))

(test x)			; prints 5

x					; gives 3



Is Python call-by-value?

x = 3

def test(y):
	y = 5
	print(y)

test(x)				# prints 5

x					# prints 3



But is Python really call-by-value?

x = [1, 3, 5, 7]

def test(y):
	y[0] = 5
	print(y)

test(x)				# prints [5, 3, 5, 7]

x					# gives [5, 3, 5, 7]



So should we do that test in Scheme too?

(define x '(1 3 5 7))

(define test
  (lambda (y)
  	(set! y (cons 5 (cdr y)))
    (display y)))

(test x)			; prints (5 3 5 7)

x					; gives (1 3 5 7)



But maybe that wasn't a good test?

(define x '(1 3 5 7))

(define test
  (lambda (y)
    (set-car! y 5)
    (display y)))

(test x)			; prints (5 3 5 7)

x					; gives (5 3 5 7)