問題2.2

SICP

線分のデータ構造。特に考える必要なし。

(define (make-segment start end)
  (cons start end))
(define (start-segment line)
  (car line))
(define (end-segment line)
  (cdr line))
(define (make-point x y)
  (cons x y))
(define (x-point point)
  (car point))
(define (y-point point)
  (cdr point))

(define (center a b)
  (/ (+ a b) 2))
(define (midpoint-segment line)
  (let ((x (center (x-point (start-segment line))
		   (x-point (end-segment line))))
	(y (center (y-point (start-segment line))
		   (y-point (end-segment line)))))
	(cons x y)))

p.s. Undoしようとして、間違って、『Option+z』を押したら、オメガが出てきた。知らんカッター。