Simple functor example in Standard ML

list.sml

(*
 * $ polyc list.sml
 * $ ./a.out
 * 2
 * 2
 *)

signature LIST =
sig
    type t
    type element
    val indexOf : (t * element * int) -> int
end

signature LIST_DOMAIN =
sig
    eqtype t
end

functor List (D: LIST_DOMAIN) : LIST where type element = D.t =
struct
    type element = D.t
    type t = element list

    fun indexOf (l: t, e: element, start: int) : int =
        case l of
            [] => ~1
          | (head :: tail) =>
        case head = e of
            true => start
          | false => indexOf (tail, e, start + 1)
end

structure StringT =
struct
    type t = String.string
end

structure IntT =
struct
    type t = Int.int
end

structure StringList = List (StringT)

structure IntList = List (IntT)

fun main () =
    let
        val i = StringList.indexOf (["foo", "bar", "car"], "car", 0)
        val j = IntList.indexOf ([1, 2, 5, 7], 5, 0)
    in
        PolyML.print (i);
        PolyML.print (j);
        ()
    end