Re: [Caml-list] ocambuild vs ocamldep circular dependencies

[Jonathan Protzenko <jonathan.protzenko@gmail.com>]

FYI, here's how the source code of OCaml deals with this very problem.

https://github.com/ocaml/ocaml/blob/trunk/typing/typecore.ml#L84 
(forward reference...)

https://github.com/ocaml/ocaml/blob/trunk/typing/typemod.ml#L1564 
(filled later on).

Arguably a dirty hack, but works really nicely in practice imho. Works 
with OCamlbuild too.

~ jonathan

On 7/9/16 9:16 AM, Petter A. Urkedal wrote:
> There is a way to do it if I understood the problem right by
> functorizing and linking explicitly with the `module rec` construct:
>
> $ cat first_sig.mli
> module type S = sig
>    type a
>    type b
>    val null : unit -> a
>    val succ : b -> a
> end
>
> $ cat second_sig.mli
> module type S = sig
>    type a
>    type b
>    val null : b
>    val succ : a -> b
> end
>
> $ cat first.ml
> module Make (X : Second_sig.S) = struct
>    type b = X.b
>    type a = N | B of b
>    let null () = N
>    let succ x = B x
> end
>
> $ cat second.ml
> module Make (X : First_sig.S) = struct
>    type a = X.a
>    type b = N | A of a
>    let null = N
>    let succ x = A x
> end
>
> $ cat both.ml
> module rec A : First_sig.S with type b = B.b = First.Make (B)
>         and B : Second_sig.S with type a = A.a = Second.Make (A)
>
> There are typically some shared types at the top of the signatures,
> which could be put into a separate file and included.
>
>
> On 9 July 2016 at 17:16, Soegtrop, Michael <michael.soegtrop@intel.com> wrote:
>> Dear Gabriel,
>>
>>
>>
>> The current implementation happens to allow them, but this does not mean
>> that they are "valid programs".
>>
>> I see what you mean: since I can’t put both modules into a single file
>> without changing the two let into let rec / and or using recursive modules,
>> it is not valid OCaml, although the compiler accepts it if presented
>> separately.
>>
>>
>>
>> Let me see what I can do here. The mutual dependent part is a large part of
>> both modules so it wouldn’t be very nice to put them into one file. But if
>> it is the only option to get a valid OCaml program, I will look into it.
>>
>>
>>
>> Still I think it is unfortunate that OCaml doesn’t support this. It makes it
>> harder to structure software in an aspect oriented way. For simple data
>> structures it is fine to bundle all aspects into a single module or file.
>> For a data structure like a C AST, the module / file will get quite large. I
>> can separate off the statements easily, but this is only a small part of the
>> AST. The largest part are expressions and types, and these are recursively
>> dependent on each other.
>>
>>
>>
>> Best regards,
>>
>>
>>
>> Michael
>>
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