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FAQ - Module language

Can I have two mutually recursive compilation units / structures / signatures / functors?
Currently not in the stable langage. However there exists an OCaml extension (which is subject to change or removal at any time) which adresses some of these problems.
How do I express sharing constraints between modules?
Use manifest type specifications in the arguments of the functor. For instance, assume defined the following signatures:
module type S1 = sig ... type t ... end
module type S2 = sig ... type u ... end
To define a functor F that takes two arguments X:S1 and Y:S2 such that X.t and Y.u are the same, write:
module F (X: S1) (Y: S2 with type u = X.t) =
  struct ... end
Indeed, internally this expands to
module F (X: S1) (Y: sig ... type u = X.t ... end) =
  struct ... end
Compilation units are forced to be modules. What if I want to make a unit with a functor or a signature instead?
In OCaml, functors and signatures (module types) can be components of modules. So, just make the functor or signature be a component of a compilation unit. A good example is the module Set from the standard library.