なんか rank2-polymorphism と existential type が欲しかったけど無かったので first-class module を使った結果がこれだよ

rank2polymorphism_and_existentialttype.ml

type 'a node =
  | A: int -> int node
  | B: string -> string node

module type Exists = sig
  type t
  val ret: t node
end

module type PolyFun = sig
  val f: 'a node -> (module Exists)
end

let f: type a. a node -> (module Exists) = function
  | A _ -> let module Ret = struct type t = string let ret = B "hoge" end in (module Ret)
  | B _ -> let module Ret = struct type t = int    let ret = A 0 end in (module Ret)

let g: type a. a node -> (module PolyFun) -> (module Exists) = fun n pf ->
  let module PF = (val pf: PolyFun) in
  PF.f n

let result = g (A 1) (module struct let f = f end: PolyFun)