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Working on dependent projects with ocamlbuild

If you are on a file system that supports symbolic links, the following setup allows to work simultaneously on a base project and two independent projects p1 and p2 that depend on base. The setup is very simple and any change to the sources of base automatically gets propagated in the builds of p1 and p2.

Suppose the sources are distributed as follows :

base/src
p1/src
p2/src

Simply create these symbolic links :

ln -s ../base/src p1/base
ln -s ../base/src p2/base

and modify p1 and p2's _tags files as follows :

echo "<base>: include" >> p1/_tags
echo "<base>: include" >> p2/_tags

The rest will be sorted out by ocamlbuild. When a change is done in base/src nothing needs to be recompiled there: new builds of p1 or p2 will automatically use the latest version of base's sources.

Internal and external project interface

Sometimes in the base project you want to have both an internal and an external interface for module definitions. For example in the following layout assume b.ml accesses definitions in a.ml that clients of the base project should not see.

base/src/a.ml
base/src/b.ml

In order to do this, adjust the directory layout to :

base/src/internal/a.ml
base/src/internal/b.ml
base/src/base.ml
base/src/base.mli

With base.ml as follows :

module A = A
module B = B

and base.mli restricting the signatures of A and B as needed. Finally add the following p1/myocamlbuild.ml and p2/myocamlbuild.ml plugins.

open Ocamlbuild_plugin;;
open Ocamlbuild_pack;;
dispatch begin function
| After_rules ->
    Pathname.define_context "base" ["base/internal"]
| _ -> ()
end;;

Caveats

  • Scalability. Since both p1 and p2 build their own version of base, this means longer build times after an ocamlbuild -clean. On the other hand with ocamlbuild you don't need to clean as much as you had with makefiles.

  • If base/src needs a plugin to build you will have to integrate it into p1 and p2's myocamlbuild.ml. This caveat may be removed in the future once ocamlbuild supports multiple plugins. Note that if you are only using tagged caml sources nothing special needs to be done, just put your tags in base/src/_tags.