sicp 2.4

Exercise 2.4.  Here is an alternative procedural representation of pairs. For this representation, verify that (car (cons x y))yields x for any objects x and y.

 

 

(define (cons x y)
  (lambda (m) (m x y)))

(define (car z)
  (z (lambda (p q) p)))

 

What is the corresponding definition of cdr? (Hint: To verify that this works, make use of the substitution model of section 1.1.5.)

 

(define (cons x y)
  (lambda (m) (m x y)))

(define (car z)
  (z (lambda (p q) p)))

(define (cdr z)
  (z (lambda (p q) q)))

 

(cdr (cons 1 2))

->((cons 1 2) (lambda (p q) q))

->((lambda (m) (m 1 2)) (lambda (p q) q))

->((lambda (p q) q) 1 2)

->2