I was hoping for some feedback like this. You're right, I think I could solve it by using an ADT and splitting `apply` into two parts: atom-ops and list-ops.

  module LispData = struct
    (* eventually support more than just strings as ints *)
    type t = Atom of string | List of string list
    let create v = (v, if is_atom v then `Atom else `List)
    let recover (v, t) : t = if t = `Atom then as_atom v else as_list v
  end

  let rec apply car cdr = match LispData.recover car with
    | atom `Atom -> atom_ops atom cdr
    | list `List -> list_ops list cdr

This is what I'm thinking, roughly speaking.