帮助sml - 困扰

Question

如何声明一个函数，使Listn：''列表 - ＆gt; ''列表 - ＆gt; bool，listn xs和ys返回true。

示例：lisen [＃“1”＃“2”]，[＃“1”“＃3”]返回false和[＃“1”，＃“2”] [＃“2”，＃“1 “]返回true

Answer 1

试试这个：

infix member
fun x member []         = false
  | x member (y::ys)    = x = y orelse x member ys;

fun listn (x::xs) ys = x member ys andalso listn xs ys
  | listn [] _       = true;