module type Base = sig ... end
module Unit : Base with type t = unit
module String : Base with type t = string
module Blob : Base with type t = string
module Boolean : Base with type t = bool
module Integer : Base with type t = int
module Long : Base with type t = int
module Double : Base with type t = float
module Float : Base with type t = float
module DateTime : Base with type t = CalendarLib.Calendar.t