Re: [Caml-list] Another question about modules

[Martin Jambon <martin.jambon@ens-lyon.org>]

On Tue, 15 Jul 2008, Andre Nathan wrote:

> Hi
>
> I've run this:
>
> ocamlc -c a.mli
> ocamlc -c b.mli
> ocamlopt -c a.ml
>
> The third command gives the error. I thought that the circular
> dependency problem was related only to mutually-dependent types on
> separate modules, but I guess I was wrong.
>
> Searching the archives, it seems that the solution is to eliminate the
> references to B in A by passing a function argument to A.f, as in
>
>  type t = { id: int }
>  let f bfun x = print_int x.id; bfun x
>
> and then in b.ml something like
>
>  let f x = print_endline (string_of_int 42)
>  let _ = let a = { A.id = 1 } in A.f f a
>
> That appears to solve the issue, although in my actual code it means
> adding an extra parameter to many functions, since the call to what was
> B.f here is somewhat deep in the call stack, so maybe there is a better
> solution.

If you have no other choice, you can do something like that:

--- a.ml ---

let b_fun_ref = ref (fun _ -> assert false)
let b_fun x = !b_fun_ref x

let a_fun x =
   ...
   b_fun
   ...

--- b.ml ---

let b_fun x =
  ...
  A.a_fun
  ...

let () =
   A.b_fun_ref := b_fun



For complex projects, the object system happens to be an excellent way of 
avoiding interdependency problems:

* You define class types early. Not a requirement, but highly recommended.
* You develop independent modules that would work with objects of these
   class types, without worrying about inter-dependencies.
* You just pass objects around, since they carry all the functions needed.
* You can have different implementations of the same class type.
* You can use inheritance.

In my opinion, using OCaml objects makes sense only in large 
projects. It's not theoretically or algorithmically exciting, but makes 
things manageable without magic skills.

I would say that objects are not well-suited to represent nodes 
of low-level data structures, because of performance but 
also because you may want to use pattern matching, circularity and other 
things that tuples, records and variants do better.


I hope you'll find this useful
:-)


Martin


> Thanks,
> Andre
>
> On Tue, 2008-07-15 at 20:18 -0400, Ashish Agarwal wrote:
>> Firstly, you have a circular dependency. How are you compiling? That
>> should be the first error you get.
>>
>>
>> On Tue, Jul 15, 2008 at 6:51 PM, Andre Nathan <andre@digirati.com.br>
>> wrote:
>>         I think this is similar to this simpler problem:
>>
>>         a.ml:
>>
>>          type t = { id: int }
>>          let f x = print_int x.id; B.f x
>>
>>         a.mli:
>>
>>          type t
>>          val f : t -> unit
>>
>>
>>         b.ml:
>>
>>          let f x = print_int 42
>>
>>         b.mli:
>>
>>          val f : A.t -> unit
>>
>>
>>         Which results in "This expression has type t but is here used
>>         with type
>>         A.t" in a.ml, even though t and A.t are the same type. Is
>>         there a
>>         general solution for this kind of situation?
>>
>>         Thanks,
>>
>>         Andre
>>
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>
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