Use external SML file in SML/NJ build configuration

Question

I have this configuration that works but my attempt to use an external file use "external.file"; in the functor failed. I wanted to test an external file by calling the functions in the functor since I didn't want to touch the external file. It doesn't export any modules.

When I listed the file after is it reported an error as no modules were exported. Believe this file should be included in the configuration somewhere else. How can I do that ?

build.cm

  source (-)       (* export all defined modules *)

  structure Test(* OR, export selectively *)
  signature THEOREM
  functor Theorem
is
  (* Import the SML standard library, aka Basis.  *)
  (* See: http://sml-family.org/Basis/ *)
  $/basis.cm

  (* Import the SML/NJ library *)
  (* Provides extra data structures and algorithms. *)
  (* See: https://www.smlnj.org/doc/smlnj-lib/Manual/toc.html *)
  $/smlnj-lib.cm

  (* List each source file you want to be considered for compilation. *)
  ./test.sml
  ./theorem.sig
  ./theorem.fun

theorem.fun

structure Test =

  struct

    structure T = Theorem()


    fun main () =
       T.update


end

theorem.sig

signature THEOREM =
  sig
    val message : string

    val update : (int * string) list option
  end

theorem.fun

The update' function is supposed to be in the external file. I copied it here because I couldn't use that file.

functor Theorem() : THEOREM=
  struct

    fun update'  front [] (l,n) = NONE
  |  update'  front ((l',n')::pairs) (l,n) =
     if l=l' then
         SOME(front @ ((l,n)::pairs) )
     else
         update' ((l',n')::front) pairs (l,n)

    val update = update' [] [(1, "one")] (1,"one")

 end

I also read the CM documentation