$> cat print.sml
fun printList ls =
  print ("[" ^ String.concatWith ", " ls ^ "]\n");

$> cat stack.sml
signature Stack =
sig
    type α Stack

    val empty   : α Stack
    val isEmpty : α Stack → bool

    val cons    : α × α Stack → α Stack
    val head    : α Stack → α
    val tail    : α Stack → α Stack
end

structure List : Stack =
stack
  type α Stack = α list

  val empty = []
  fun isEmpty s = null s

  fun cons (x,s) = x ∷ s
  fun head s = hd s
  fun tail s = tl s
end

$> cat stack-example.sml
use "stack.sml";
use "print.sml";
printList (tail [1,2,3]);

----------------------------------------
$> mlton stack-example.sml
Error: stack-example.sml 1.1.
  Undefined variable use.
Warning: <bogus>.
  Unable to locally determine type of variable: it.
    type: ???
    in: val it = use "stack.sml"
Error: stack-example.sml 2.1.
  Undefined variable use.
Warning: <bogus>.
  Unable to locally determine type of variable: it.
    type: ???
    in: val it = use "print.sml"
Error: stack-example.sml 3.1.
  Undefined variable printList.
Error: stack-example.sml 3.12.
  Undefined variable tail.
Warning: <bogus>.
  Unable to locally determine type of variable: it.
    type: ???
    in: val it = printList (tail [1, 2, 3])
compilation aborted: parseAndElaborate reported errors