Using findlib and a syntax extension with ocamlbuild

["Paolo Donadeo" <p.donadeo@gmail.com>]

I'm fighting again with ocamlbuild. In the (excellent!) wiki I found
two useful recipes: how to use findlib with ocamlbuild and how to
compile source code using a syntax extension. Now it happens that I
have both these needs, and I'm not so sure how to mix the ingredients
and obtain a good dish.

At the moment the solution I have discovered (by trial and error)
works well, but it doesn't seem very clean.

The involved files in my source directory are these:

./xnuncc.ml
./myocamlbuild.ml
./ext_libs/openin-20070524/pa_openin.ml
./_tags

In xnuncc.ml I use the openin extension, and xnuncc.ml is the main
compilation unit.

Here is the contents of _tags and the plugin for ocamlbuild:

======= 8< CUT HERE: _tags >8 =======
<dom> or <files> or <fragment_composer> or <test> or <doc> : include
<ext_libs/openin-20070524> : include
true : dtypes
<**/pa_openin.ml> : camlp4of, use_camlp4of
"xnuncc.ml" : camlp4o, use_openin
======= 8< END HERE: _tags >8 =======

======= 8< CUT HERE: myocamlbuild.ml >8 =======
open Ocamlbuild_plugin;;
open Command;;

let packages = "str,netstring,oUnit,extlib";;

let ocamlfind cmd = S[A"ocamlfind"; A cmd; A"-package"; A packages];;

flag ["ocaml"; "link"] (A"-linkpkg");;

dispatch begin function
  | After_options ->
      Options.ocamlc := ocamlfind "ocamlc";
      Options.ocamlopt := ocamlfind "ocamlopt";
  | After_rules ->
      ocaml_lib ~extern:true ~dir:"+camlp4" "camlp4of";
      flag ["ocaml"; "pp"; "use_openin"]
(A"ext_libs/openin-20070524/pa_openin.cmo");
      dep ["ocaml"; "ocamldep"; "use_openin"]
["ext_libs/openin-20070524/pa_openin.cmo"];
  | _ -> ()
end
======= 8< END HERE: myocamlbuild.ml >8 =======

As I wrote this works, but the aspect I don't like is why I have to
specify the full path of "ext_libs/openin-20070524/pa_openin.cmo"
instead of a simple "pa_openin.cmo", inside myocamlbuild.ml. Including
the directory "ext_libs/openin-20070524" in the _tags file does't seem
to be enough.

If I use "pa_openin.cmo", indeed, the result is an absurd (for me)
error message starting with:

======= 8< CUT HERE: error message >8 =======
Ocamlbuild knows of no rules that apply to a target named
ext_libs/openin-20070524/xnuncc.d.ml. This can happen if you ask
Ocamlbuild to build a target with the wrong extension (e.g. .opt
instead of .native) or if the source files live in directories that
have not been specified as include directories.
Backtrace:
  - Failed to build the target xnuncc.d.byte
      - Failed to build all of these:
          - Building ext_libs/openin-20070524/xnuncc.d.byte:
              - Failed to build all of these:
                  - Building ext_libs/openin-20070524/xnuncc.d.cmo:
                      - Failed to build all of these:
                          - Building ext_libs/openin-20070524/xnuncc.d.ml
                          - Building ext_libs/openin-20070524/xnuncc.ml:
                              - Failed to build all of these:
                                  - Building ext_libs/openin-20070524/xnuncc.mly
... ... ...
======= 8< END HERE: error message >8 =======

At the end of the story the question is: is this solution the best
possible for compiling a project with findlib and a syntax extension
present in a subdir of the project?

TIA,

-- 
                                            Paolo


Paolo Donadeo, Senior Software Engineer
Studio Associato 4Sigma
Email: p.donadeo@4sigma.it
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:wq