draw-eval-apply.scm

(define H 300)
(define r (/ H 2))

(define (ypos x)
  (if (<= x 1)
      (/ (sqrt (- 1 (sqr (- (* 2 x) 1)))) 2)
      (/ (sqrt (- 1 (sqr (- (* 2 x) 3)))) -2)))
(define (ypos-as-posn x)
  (make-posn (* x r)
             (* (+ 1 (ypos x)) r)))
(define (div-by n)
  (lambda (x) (/ x n)))

(define (range start stop)
  (if (<= start stop)
      (cons start (range (+ 1 start) stop))
      (list)))
(define (do-pairwise action lst)
  (if (>= (length lst) 2)
      (begin (action (car lst) (cadr lst))
             (do-pairwise action (cdr lst)))
      'done))

(start H H)
(draw-circle (make-posn r r) r 'black)
(do-pairwise (lambda (a b) (draw-solid-line a b 'black))
             (map ypos-as-posn
                  (map (div-by r) (range 0 H))))
(draw-solid-string (make-posn (* r 0.3) r) "EVAL")
(draw-solid-string (make-posn (* r 1.3) r) "APPLY")