Higher-order relabelling function, ported from OCaml to Standard ML

mk_idmap.sml

exception NotFound

fun mk_idmap (mk_id : unit -> 'b) : ''a list -> ''a -> 'b =
  foldl
    (fn (x, acc) =>
      let val new_id = mk_id () in
        fn id => if id = x then new_id else acc id
      end)
    (fn _ => raise NotFound)

val mk_id : unit -> int =
  let val id = ref 0 in
    fn () => (id := !id + 1; !id)
  end