[Caml-list] conditional compilation and ocamlbuild

[Pietro Abate <Pietro.Abate@pps.jussieu.fr>]

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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