幫助sml - 遇到困難

Question

如何聲明一個函數如此listn：''列表 - >''列表 - > bool，listn xs和ys返回true。例如：lisen [＃「1」＃「2」]，[＃「1」「＃3」]返回false，[＃「1」，＃「2」] [＃「2」，＃「 1" ]返回true

Answer 1

試試這個：

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;