SI units in OCaml

module SI : sig
  type 'a t
  type meter
  type kilogram
  type second

  type ('a, 'b) mul
  type ('a, 'b) div

  val meters : float -> meter t
  val kilograms : float -> kilogram t
  val seconds : float -> second t

  val (+!) : 'a t -> 'a t -> 'a t
  val (-!) : 'a t -> 'a t -> 'a t
  val ( *! ) : 'a t -> 'a t -> ('a, 'b) mul t
  val (/!) : 'a t -> 'a t -> ('a, 'b) div t

  val to_float : 'a t -> float
end = struct
  type 'a t = SiUnit of float
  type meter
  type kilogram
  type second

  type ('a, 'b) mul
  type ('a, 'b) div

  let meters x = SiUnit x
  let kilograms x = SiUnit x
  let seconds x = SiUnit x

  let (+!) (SiUnit x) (SiUnit y) = SiUnit (x +. y)
  let (-!) (SiUnit x) (SiUnit y) = SiUnit (x -. y)

  let ( *! ) (SiUnit x) (SiUnit y) = SiUnit (x *. y)
  let (/!) (SiUnit x) (SiUnit y) = SiUnit (x /. y)

  let to_float (SiUnit x) = x
end