type unique = unit ref

type ty = 
  | INT
  | STRING
  | RECORD of (Symbol.t * ty) list * unique
  | ARRAY of ty * unique
  | NIL
  | UNIT
  | NAME of Symbol.t * ty option ref

let rec ty_s = function
  | INT -> "int"
  | STRING -> "string"
  | RECORD _ -> "record"
  | ARRAY _ -> "array"
  | NIL -> "nil"
  | UNIT -> "unit"
  | NAME (_, k) ->
      (match !k with
      | None -> failwith "error"
      | Some k -> ty_s k)