; Re-implementation of cons-stream and cdr-stream (which are
; predefine in scm.)

(define-macro (cons-stream head tail) 
    `(cons ,head (lambda () ,tail)))

(define (cdr-stream str)
    (if (procedure? (cdr str)) 
        ; Compute and memoize tail
        (set-cdr! str ((cdr str))))
    (cdr str))

;; All 1's
(define ones (cons-stream 1 ones))

;; Sums of corresponding elements of infinite streams a and b
(define (add-streams a b) 
    (cons-stream (+ (car a) (car b))
                 (add-streams (cdr-stream a) (cdr-stream b))))

;; The stream 1 2 3 ...
(define nums (cons-stream 1 (add-streams ones nums)))

;; The Fibonacci sequence
(define fib (cons-stream 1 ; Alternatively, start with 0.
               (cons-stream 1 
                  (add-streams fib (cdr-stream fib)))))