Compiling with ocamlbuild and camlp5 - preprocessor

Question

I am trying to make a project I am working on compile with ocamlbuild, in order to avoid the use of a regular Makefile, which I find to be slightly more complicated.
Specifically, I have a syntax extension file (extend.ml), which I need to be compiled first. In a regular Makefile there would be a rule:
extend.cmo: extend.ml
$(OCAMLC) -pp "camlp5o pa_extend.cmo q_MLast.cmo" -I +camlp5 -c $<
and then, for calculating the dependencies there would be a rule like this:
depend: $(MLFILES) extend.cmo
$(OCAMLDEP) -pp "camlp5o ./extend.cmo"
Of course, the creation of any object file would require a similar rule to the one above.
My question is, how can I integrate these rules/requirements to one ocamlbuild command (if possible)?
I have tried to compile the extend.ml file first, and then use the following command:
ocamlbuild -pp "camlp5o ./extend.cmo" -I +camlp5 -use-menhir -no-hygiene Main.byte
but I don't think it's optimal in any way.
Unfortunately, I am not familiar with the use of ocamlbuild as a compilation tool, so any help will be much appreciated.

Answer 1

You could define two new tags, compile_extend and use_extend, that specify the expected options. In your myocamlbuild.ml file:
open Ocamlbuild_plugin
let my_flags () =
flag ["ocaml"; "pp"; "compile_extend"]
(S [A"camlp5o"; A "pa_extend.cmo"; A "q_MLast.cmo"]);
flag ["ocaml"; "pp"; "use_extend"]
(S [A"camlp5o"; A "extend.cmo"]);
(* files with the use_extend flag must depend on extend.cmo *)
dep ["ocaml"; "use_extend"] ["extend.cmo"];
()
let () =
dispatch (function
| After_rules ->
my_flags ();
| _ -> ())
Then you would have a tags file with:
"extend.cmo": compile_extend
<Main.*>: use_extend
That said, this is all bling guessing, I have not tested this setup. Could you provide a tarball with an example extend.ml file and Main.ml allowing to reproduce your situation?