It's a hack, but have you considered symlinking common.ml to a different
name (common_without_distrue, no idea) to bypass the per-name tag
attribution and caching logic of Ocamlbuild?
You could also generate files on the fly (add ocamlbuild rules to generate
common_for_a.ml and common_for_b.ml, then have a and b depend on those
modules).

The problem is that it changes the name of the Common module for a.ml and
b.ml.

On Thu, Jun 2, 2011 at 11:38 AM, Pietro Abate
<Pietro.Abate@pps.jussieu.fr>wrote:

> Hello list,
>
> I've three modules a.ml, b.ml and common.ml. A and B both
> use Common, but Common is compiled with Camlp4MacroParser
> to exclude/include part of the code.
>
> The problem I want to include some part of the code when
> I compile A and exclude it when I compile B.
>
> so the canonical way to use Camlp4MacroParser is to add a rule like
>
> <common.ml>: pp(camlp4o Camlp4MacroParser.cmo -DISTRUE)
>
> and when I compile everything I get something like :
>
> $ocamlbuild -classic-display b.byte a.byte
> /usr/bin/ocamldep -modules b.ml > b.ml.depends
> /usr/bin/ocamldep -pp 'camlp4o Camlp4MacroParser.cmo -DISTRUE' -modules
> common.ml > common.ml.depends
> /usr/bin/ocamlc -c -pp 'camlp4o Camlp4MacroParser.cmo -DISTRUE' -o
> common.cmo common.ml
> /usr/bin/ocamlc -c -o b.cmo b.ml
> /usr/bin/ocamlc common.cmo b.cmo -o b.byte
> /usr/bin/ocamldep -modules a.ml > a.ml.depends
> /usr/bin/ocamlc -c -o a.cmo a.ml
> /usr/bin/ocamlc common.cmo a.cmo -o a.byte
>
> now, both A and B use common.ml compiled with -DISTRUE .
> But this is not what I want.
>
> I'm pretty sure I can't use _tags in this situation. But I'm
> not sure how to write a simple myocamlbuild rule to basically
> creating two common.cmo.
>
> One for A as
> /usr/bin/ocamlc -c -pp 'camlp4o Camlp4MacroParser.cmo -DISTRUE' -o
> common.cmo common.ml
>
> and one for B as
> /usr/bin/ocamlc -c -pp 'camlp4o Camlp4MacroParser.cmo' -o common.cmo
> common.ml
>
> -----------a.ml -------
> let a = print_endline (Common.func ())
>
> -----------b.ml -------
> let a = print_endline (Common.func ())
>
> ----------common.ml -------
> let func () =
> IFDEF ISTRUE THEN
> "is true"
> ELSE
> "is not true"
> END
>
> I guess the idea is to add a rule that says :
> if you are compiling A then run the preprocessor with -DISTRUE otherwise
> without it. But doing this I want also to force ocamlbuild not to reuse an
> existing common.cmo that maybe was compiled from a "different" source...
>
> Of course my real problem is a bit more complicated as in the conditional
> compilation process I exclude/include calls an external C library and
> I want to avoid linking this external C library to binaries that do not use
> it at all ...
>
> pietro
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