Functors taking recursive module signature as argument

Learning
2

It’s rare to really need recursive modules, if you can share more code we could advice if they are worth the trouble…

Otherwise, it should be possible to break down the issue in two steps:

module A (B : sig type t end) = struct
  type t = Wrap of B.t

  module Make (Bfun : sig val to_a : B.t -> t end) : sig
    val to_b : t -> B.t
  end = struct
    let to_b (Wrap b) = b
  end
end

module A_int = A (struct type t = int end)
module A_fun = A_int.Make (struct let to_a b = A_int.Wrap b end)