walker...to the next step

why functional programming matters里看来的例子，改成了Scheme版本。

占个位子。等SICP做到第四章，搞出LE，这个就可以解决了。

(define (within eps lst)
  (let ((a (car lst))
        (b (cadr lst)))
    (if (<= (abs (- a b)) eps)
        b
        (within eps (cdr lst)))))

(define (repeat f a)
  (cons a (repeat f (f a))))

(define (square-root guess eps n)
  (within esp (repeat
               (lambda (x) (/ (+ x (/ n x)) 2))
               guess)))