Excerpts from Hezekiah M. Carty's message of Mon Sep 17 20:08:49 +0200 2007:
> On Mon, 17 Sep 2007, Nicolas Pouillard wrote:
> 
> > > I have also tried creating a myocamlbuild.ml to get around this, but
> > > using something like this in myocamlbuild.ml:
> > > 
> > > flag ["use_openin"]
> > >   (S [A "-I"; A "+camlp4"; A "-pp"; A "camlp4o pa_openin.cmo"]);;
> > 
> > You  certainly don't want to add these flags to any commands that treat a file
> > that  is  tagged  use_openin.  You  certainly  want to add them only for ocaml
> > preprocessing.
> > 
> > flag ["ocaml"; "pp"; "use_openin"]
> >      (A"pa_openin.cmo");;
> > 
> 
> This gets me a little bit closer I think, but I'm still not able to
> make ocamlbuild do what I want.
> 
> Here's the current code -
> test.ml:
> let sum l = open List in fold_left (+) 0 l

Ok

> _tags:
> "test.ml": use_openin

Your second attempt is good you need the camlp4o tag.

> myocamlbuild.ml:
> open Ocamlbuild_plugin;;
> open Command;;
> flag ["ocaml"; "pp"; "use_openin"] (A "pa_openin.cmo");;

You absolutely need to wrap this flag declaration to avoid order of initialization problems (see below).

> Using the above "flag ..." line adds "-pp pa_openin.cmo" to the
> compilation command line using the following line in _tags:
> "test.ml": use_openin
> 
> If I change the _tags line to:
> "test.ml": camlp4o, use_openin

Ok

> the output from ocamlbuild test.cma is:
> + /home/hcarty/Applications/godi/bin/ocamldep.opt -pp 'pa_openin.cmo
> camlp4o' -modules test.ml > test.ml.depends
> sh: pa_openin.cmo: command not found
> Preprocessing error on file test.ml

That's due to the order of initialization.

> How do I fix this to properly use a syntax extension such as
> pa_openin.cmo and be able to specify the location of the .cmo, but
> only for the files which need that specific extension?  ie. add "-I
> +camlp4" to the ocaml* command line, and have the -pp value in the
> correct order.

$ cat myocamlbuild.ml
open Ocamlbuild_plugin;;
open Command;;
dispatch begin function
| After_rules ->
    flag ["ocaml"; "pp"; "use_openin"] (A"pa_openin.cmo");
    dep ["ocaml"; "ocamldep"; "use_openin"] ["pa_openin.cmo"];
| _ -> ()
end;;

> I apologize for asking such a basic question, but I haven't been
> able to figure out the answer from the user's guide or other
> examples I've found.  Once this is all worked out I would be happy
> to add this to a wiki somewhere.

Yes you're right!

I've quickly added a page on that:

http://brion.inria.fr/gallium/index.php/Ocamlbuild

-- 
Nicolas Pouillard aka Ertai