The magic of extensible types.

tag.sml

signature TAG =
sig
  type tagged
  type 'a tag

  val new    : unit -> 'a tag
  val tag    : 'a tag -> 'a -> tagged
  val untag  : 'a tag -> tagged -> 'a option
end

functor Tag () :> TAG =
struct
  type tagged = exn
  type 'a tag = { handleTag : exn -> 'a option
                , makeTag   : 'a -> exn
                }

  fun new () : 'a tag =
    let
      exception Tag of 'a
    in
      { makeTag = Tag
      , handleTag = fn e =>
          (raise e) handle Tag a => SOME a |  _ => NONE
      }
    end

  fun tag {makeTag, handleTag} = makeTag
  fun untag {makeTag, handleTag} = handleTag
end

(* A quick demonstration *)
local
  structure Test = Tag ()
in
  open Test
end

val intTag = new () : int tag
val stringTag = new () : string tag

val i = tag intTag
val s = tag stringTag

val mixed = [i 1, i 2, s "Hello World", s "Foo"]
val intsOnly = List.mapPartial (untag intTag) mixed
val stringsOnly = List.mapPartial (untag stringTag) mixed