Recursive types in nested modules

Modules can be recursive as well, but then you need to give them explicit module type definitions:

module Foo = {
  module rec A: {
    type t = {b: B.t}
  } = {
    type t = {b: B.t}
  }

  and B: {
    type t = {a: A.t}
  } = {
    type t = {a: A.t}
  }
}