Your solution is quite good and follows ocamlbuild principles.

Excerpts from Erick Tryzelaar's message of Mon Aug 06 12:11:16 +0200 2007:
> erickt@dslextreme.com wrote:
> > I was trying to adapt the parser dypgen (http://dypgen.free.fr/)'s build
> > system to use ocamlbuild, but I ran into a problem. The final dypgen
> > grammar is generated by an internal intermediary generator called pgen.
> > The problem I'm having is that I don't know how to get ocamlbuild to
> > automatically build pgen before we can process a %.dyp file. I suppose I
> > could do this in two separate calls to ocamlbuild, but I feel like this
> > can be done using a plugin. Is this possible?
> 
> For those that are interested, here's the solution I came up with. We 
> use the "build" function we get in the rule to dynamically dispatch the 
> building of the parser if we haven't generated it yet:
> 
> myocamlbuild.ml:
> dispatch begin function
>   | After_rules ->
>       rule "dypgen: .dyp -> .ml & .mli"
>         ~deps:["%.dyp"]
>         ~prods:["%.ml"; "%.mli"]
>         begin fun env build ->
>           let dyp = env "%.dyp" in
>           let tags = tags_of_pathname dyp++"ocaml"++"parser" in
>          
>           (* Since pgen and dypgen programs use the same extension, we need
>            * to be able to switch between the two. Do this by tagging 
> the pgen
>            * files with "use_pgen".
>            * I'm not sure how we should do byte/native, so we'll just 
> use the
>            * native. *)
>           let dypgen, tags =
>             if Tags.mem "use_pgen" tags
>             then "dyp/generators/pgen/pgen.native", tags++"pgen"
>             else "dyp/generators/dypgen/dypgen.native", tags++"dypgen"
>           in
>            
>           (* try to build the parser if it doesn't exist yet *)
>           List.iter begin function
>             | Outcome.Good o -> ()
>             | Outcome.Bad exn -> raise exn
>           end (build [[dypgen]]);

You can use the Outcome.ignore_good function.

List.iter Outcome.ignore_good (build [[dypgen]])

>            
>           Cmd(S[A dypgen; T tags; Px dyp])
>         end;
>   | _ -> ()
> end;;
> 

-- 
Nicolas Pouillard aka Ertai