packing tokens with ocaml pattern matching

gistfile1.fs

(*
На входе список токенов, задача - объединить подпоследовательности вида "String, '_', String" и "'-', Number" в одну, т.е.
Вход: ["foo", '_', "bar", '-', 12, "some", "other", 15, "baz"]
Выход: ["foo_bar", -12, "some", "other", 15, "baz"]
*)
let pack_tokens tokens =
  let accum = ref [] in
  let rec pack tokens_left  = match tokens_left with
      [] -> List.rev !accum
    | Tokenizer.String l :: Tokenizer.Symbol '_' :: Tokenizer.String r :: tail ->
      pack (Tokenizer.String (String.concat "_" [l;r]) :: tail)
    | Tokenizer.Symbol '-' :: Tokenizer.Number n :: tail -> pack (Tokenizer.Number (-n) :: tail)
    | h :: tail -> accum := h :: !accum; pack tail
  in pack tokens