2.2 的 swift 版实现 -- 先用swift 模拟 cons car cdr

midpointSegment.swift

// mirror...
func cons<T0, T1>(_ x: T0, _ y: T1) -> (T0, T1) {
    return (x, y)
}

func car<T0, T1>(_ t:(T0, T1)) -> T0 {
    return t.0
}

func cdr<T0, T1>(_ t:(T0, T1)) -> T1 {
    return t.1
}

// sample...
//(define (make-segment start end) (cons start end))
func makeSegment<T0, T1>(_ start:T0, _ end: T1) -> (T0, T1) { cons(start, end) }
//(define (start-segment s) (car s))
func startSegment<T0, T1>(_ s: (T0, T1)) -> T0 { car(s) }
//(define (end-segment s) (cdr s))
func endSegment<T0, T1>(_ s: (T0, T1)) -> T1 { cdr(s) }
//
//(define (make-point x y) (cons x y))
func makePoint<T0, T1>(_ x:T0, _ y: T1) -> (T0, T1) { cons(x, y) }
//(define (x-point p) (car p))
func xPoint<T0, T1>(_ p: (T0, T1)) -> T0 { car(p) }
//(define (y-point p) (cdr p))
func yPoint<T0, T1>(_ p: (T0, T1)) -> T1 { cdr(p) }
//
//(define (midpoint-segment s)
//    (let ((x (/ (+ (x-point (end-segment s)) (x-point (start-segment s))) 2))
//          (y (/ (+ (y-point (end-segment s)) (y-point (start-segment s))) 2)) )
//        (make-point x y)
//    )
//)
func midpointSegment(_ s: ((Double, Double), (Double, Double))) -> (Double, Double){
    let x = ((xPoint(endSegment(s))) + (xPoint(startSegment(s)))) / 2.0
    
    let y = ((yPoint(endSegment(s))) + (yPoint(startSegment(s)))) / 2.0
    
    return cons(x, y)
}
//(define (print-point p)
//    (newline)
//    (display "(")
//    (display (x-point p))
//    (display ",")
//    (display (y-point p))
//    (display ")")
//)

func printPoint<T0, T1>(_ p: (T0, T1)) {
    print("\n(\(xPoint(p)), \(yPoint(p)))")
}

//; test case (15,15)
//(print-point (midpoint-segment (make-segment (make-point 10 10) (make-point 20 20))))
(printPoint((midpointSegment((makeSegment((makePoint(10.0, 10.0)), (makePoint(20.0, 20.0)))))))) // output: (15.0, 15.0)