A plugin for camlp4 syntax extension (pa openin)

If you just want to use a syntax extension included in an installed package, see Using ocamlfind with ocamlbuild.

Suppose you have some Camlp4 OCaml syntax extension and that you want to compile them with Ocamlbuild.

Let start by adding an Ocamlbuild plugin (required to be in a file named myocamlbuild.ml at the root of your project).

(* Open the ocamlbuild world... *)
open Ocamlbuild_plugin

(* We work with commands so often... *)
open Command

(* This dispatch call allows to control the execution order of your
   directives. *)
let () =
  dispatch begin function
    (* Add our rules after the standard ones. *)
  | After_rules ->
     (* Add pa_openin.cmo to the ocaml pre-processor when use_opening is set *)
     flag ["ocaml"; "pp"; "use_openin"] (A"pa_openin.cmo");
 
     (* Running ocamldep on ocaml code that is tagged with use_openin
        will require the cmo.  Note that you only need this
        declaration when the syntax extension is part of the sources
        to be compiled with ocamlbuild. *)
     dep ["ocaml"; "ocamldep"; "use_openin"] ["pa_openin.cmo"];
  | _ -> ()
  end

Then one can tag our files that use pa_openin:

$ cat _tags
"bar.ml": camlp4o, use_openin
<foo/*.ml> or <baz/**/*.ml>: camlp4r, use_openin
"pa_openin.ml": use_camlp4, camlp4of

Now we can compile:

$ ocamlbuild bar.byte

This will build the pa_openin.cmo and use it.