sml - 帮助 sml - 困扰

Question

如何声明一个函数，以便 Listn : ' ' a list -> ' ' a list -> bool, listn xs 和 ys 返回 true。  

示例： lisen [#"1" #"2"] , [#"1" "#3"] 返回 false 和 [#"1" , #"2"] [#"2" , #"1"] 返回真的

Answer 1

尝试这个：

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;