标准ML :如何在列表中循环？

Question

我正在尝试编写一个循环遍历列表n次的程序。假设L= a1，a2，...，我要实现的是ai+1，a i+2，...，an，a1，a2，...，ai。

我参考了上一篇关于这个问题的文章。但是，我不确定如何获得输出或ai+1，a i+2，...，an，a1，a2，...，ai。

对于输出:我尝试了-cycle(1,2,3,4，5)；

但是，我得到的错误是操作数和运算符不匹配

这是我在上一篇文章中找到的代码：

fun cycle n i = 
if i = 0 then n
else cycle (tl n) (i-1) @ [hd(n)];

Answer 1

使用if-then-else可以做到这一点：

fun cycle xs n =
    if n = 0
    then []
    else xs @ cycle xs (n - 1)

您可能希望使用模式匹配：

fun cycle xs 0 = []
  | cycle xs n = xs @ cycle xs (n - 1)

但我认为，最优雅的解决方案是使用高阶函数：

fun cycle xs n =
    List.concat (List.tabulate (n, fn _ => xs))

一个稍微困难的任务是如何为惰性列表编写一个无限循环的cycle……

datatype 'a lazylist = Cons of 'a * (unit -> 'a lazylist) | Nil

fun fromList [] = Nil
  | fromList (x::xs) = Cons (x, fn () => fromList xs)

fun take 0 _ = []
  | take _ Nil = []
  | take n (Cons (x, tail)) = x :: take (n - 1) (tail ())

local
  fun append' (Nil, ys) = ys
    | append' (Cons (x, xtail), ys) =
        Cons (x, fn () => append' (xtail (), ys))
in
  fun append (xs, Nil) = xs
    | append (xs, ys) = append' (xs, ys)
end

fun cycle xs = ...

take 5 (cycle (fromList [1,2])) = [1,2,1,2,1]在哪里。