mixin.sml

functor Member(Flag : sig type t end) =
  struct
    (* Define types and values common to both MemberA and MemberB. *)
  end

(*
 * MemberA and MemberB differ only in the declaration of Flag, the rest of
 * the types and values are the same and come from the Member functor.
 *)
structure MemberA =
  struct
    structure Flag =
      struct
        datatype t = FlagA1 | FlagA2
      end

    (* Mix in the types and values defined by Member. *)
    structure M = Member(Flag) open M
  end

structure MemberB =
  struct
    structure Flag =
      struct
        datatype t = FlagB1 | FlagB2
      end

    (* Mix in the types and values defined by Member. *)
    structure M = Member(Flag) open M
  end

val a = MemberA.Flag.FlagA1
val b = MemberB.Flag.FlagB1