list标准函数的模拟

;反序 (1 2 3) -> (3 2 1)
(define (rvs x)
  (let recur ((x x)(res '()))
    (if (null? x)
        res
        (recur (cdr x) (cons (car x) res)))))

;长度 (1 2 3) -> 3
(define (len x)
  (let recur ((x x)(y 0))
    (if (null? x)
        y
        (recur (cdr x) (+ y 1)))))

;合并列表 (x ...) (y ...) -> (x ... y ...)
(define (apd x y)
  (let recur ((x x))
    (if (null? x)
        y
        (cons (car x) (recur (cdr x))))))

;切割教程版
(define (tail x n)
  (if (zero? n)
      x
      (tail (cdr x) (- n 1))))


;引用 (1 2 3 4) 2 -> 3
(define (ref x n)
  (if (= 0 n)
      (car x)
      (ref (cdr x) (- n 1))))

;成员查询 2 (1 2 2 3) -> (2 2 3)
(define (mem i x)
  (let recur ((x x))
    (if (null? x)
        #f
        (if (equal? (car x) i)
            x
            (recur (cdr x))))))

;类字典查询 k2 ((k1 v1)(k2 v2)(k3 v3)...) ->(k2 v2)
(define (asq k x)
  (let recur ((x x))
    (if (null? x)
        #f
        (if (equal? (caar x) k)
            (car x)
            (recur (cdr x))))))


(define x '(1 2 3 4))
(define y '(5 6 7 8))

(append x y)
(apd x y)
(length x)
(len x)
(reverse x)
(rvs x)
(list-tail '(1 2 3) 1)
(tail '(1 2 3) 1)
(list-ref '(1 2 3) 2)
(ref '(1 2 3) 2)
(member 2 '(1 2 3 4))
(mem 2 '(1 2 3 4))
(assv 5 '((2 3) (5 7) (11 13))) 
(asq 5 '((2 3) (5 7) (11 13))) 

结果:

(1 2 3 4 5 6 7 8)
(1 2 3 4 5 6 7 8)
4
4
(4 3 2 1)
(4 3 2 1)
(2 3)
(2 3)
3
3
(2 3 4)
(2 3 4)
(5 7)
(5 7)
>