fun preveri(x, y) = if(String.size(x) > String.size(y)) then x else y
val l = ["aaa", "aaaaaa", "bb", "aaaa"]

fun best_string(f, xs: string list) = List.foldl f "" xs

(*Following code below doesn't work :( *)
fun preveri(x, y) = if(String.size(x) > String.size(y)) then true else false
val l = ["aaa", "aaaaaa", "bb", "aaaa"]

fun best_string(f, xs: string list) = List.foldl f "" xs