test.ml

module M = struct
  type 'a key
  type 'a value
  type t = Univ : 'a key * 'a value -> t
  type polymorphic_operations = {
    conv_key : 'a . 'a key -> string;
    conv_value : 'a . 'a value -> string;
  }
  let as_strings : type a . polymorphic_operations -> t -> (string * string) =
    fun ops t -> match t with
      | Univ (k, v) -> (ops.conv_key k, ops.conv_value v)
end