module type Symbol = sig
  type t

  val symbol : string -> t
  val name : t -> string

  type 'a table
  val empty : 'a table
  val enter : 'a table -> t -> 'a -> 'a table
  val look : 'a table -> t -> 'a option
end

module Symbol : Symbol = struct

  open Core.Std

  type t = int

  let nextsym = ref 0

  let symtbl = String.Table.create ()
  let inv_symtbl = Int.Table.create ()
  let symbol s = match Hashtbl.find symtbl s with
    | Some x -> x
    | None ->
      let sym = !nextsym
      in (nextsym := sym + 1;
          Hashtbl.set symtbl ~key:s ~data:sym;
          Hashtbl.set inv_symtbl ~key:sym ~data:s;
          sym)

  let name sym = match Hashtbl.find inv_symtbl sym with
    | Some x -> x
    | None -> ""

  type 'a table = 'a Int.Table.t
  let empty : 'a table = Int.Table.create () ~size: 4
  let enter tbl s data = Hashtbl.set tbl ~key:s ~data
  let look tbl s = Hashtbl.find tbl s
end

(*

Error: Signature mismatch:
module type Symbol =
       ...
  sig
       Values do not match:
    type t
         val empty : '_a table
    val symbol : string -> t
       is not included in
    val name : t -> string
         val empty : 'a table
    type 'a table
    val empty : 'a table
    val enter : 'a table -> t -> 'a -> 'a table
    val look : 'a table -> t -> 'a option
  end

*)