Modules from modules

makemod.ml

module type S = sig
  type t
end

module type Sout = sig
  type a
  type b
  type t = A of a | B of b
end

let make : type ta tb. (module S with type t = ta) -> (module S with type t = tb) -> (module Sout with type a = ta) =
  fun (module Ma) (module Mb) ->
    let module Mab =
      struct
        type a = Ma.t
        type b = Mb.t
        type t = A of Ma.t | B of Mb.t
      end
    in
    let m = (module Mab : Sout with type a = Ma.t) in
    m