Help sml - troubled

Question

How to declare a function so Listn : ' ' a list -> ' ' a list -> bool, listn xs and ys return true.  

Example: lisen [#"1" #"2"] , [#"1" "#3"] return false and  [#"1" , #"2"] [#"2" , #"1"] return true

Answer 1

Try this:

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;