mutually recursive modules

[Brendan Miller <catphive.lists@gmail.com>]

I'm new to ocaml.

My understanding is that the ocaml compiler does not allow for
circular dependancy of modules. What is the design reason for this? I
don't think I've ever seen a language before with this limitation. Is
it just to keep people from writing mutually recursive functions
across modules?

Suppose there are two classes, A and B defined in separate modules.
Does the no circular dependancy rule preclude a doubly linked
relationship between A and B? Let's say I want to create a doubly
linked list with alternating element types.

consider the following code, which fails to compile:

A.mli:

class c :
object
  method set_prev : B.c -> unit
  method get_prev : B.c
  method set_next : B.c -> unit
  method get_next : B.c
end


A.ml

class c =
object
  val mutable m_prev : B.c option = None
  val mutable m_next : B.c option = None

  method set_prev prev = m_prev <- prev
  method get_prev = m_prev
  method set_next next = m_next <- next
  method get_next = m_next
end;;


B.mli

class c :
object
  method set_prev : A.c -> unit
  method get_prev : A.c
  method set_next : A.c -> unit
  method get_next : A.c
end

B.ml

class c =
object
  val mutable m_prev : A.c option = None
  val mutable m_next : A.c option = None
  method set_prev prev = m_prev <- prev
  method get_prev = m_prev
  method set_next next = m_next <- next
  method get_next = m_next
end;;

This is annoying. Of course I need a way to make classes from
different modules recur. Is there a syntax for specifying an interface
to the type I'm recurring with without that type actually being in the
same module? That would be somewhat cumbersome, but would actually
more properly separate the types than the code above.

In general, could someone clarify the rational for making type
recursion so cumbersome?

thx