[SOLVED] Optional dependencies with ocamlbuild

Optional dependencies with ocamlbuild

I have an OCaml project that is currently built using OCamlMake. I am not happy with the current build system since it leaves all build artefacts in the same directory as source files, plus it also requires to manually specify order of dependencies between modules. I would like to switch to a build system that does not suffer from these issues. I decided to give Oasis a try but have run into issues.

The problems arise from the fact that the project is built in a very specific way. It supports several different database backends (PostgreSQL, MySQL, SQLite). Currently, to compile a database backend user must install extra libraries required by that backend and enable it by setting an environment variable. This is how it looks in the Makefile:

ifdef MYSQL_LIBDIR
   DB_CODE    += mysql_database.ml
   DB_AUXLIBS += $(MYSQL_LIBDIR)
   DB_LIBS    += mysql
endif

Notice that this also adds extra module to the list of compiled modules. The important bit is that there is no dependency (in a sense of module import) between any module reachable from the applications entry point and database backend module. What happens rather is that each database backend module contains top-level code that runs when a module is initiated and registers itself, using side-effects, with the main application.

I cannot get this setup to work with Oasis. I declared each of the database backend modules as a separate library that can be enabled for compilation with a flag:

Library mysql-backend
  Path          : .
  Build        $: flag(mysql)
  Install       : false
  BuildTools    : ocamlbuild
  BuildDepends  : main, mysql
  Modules       : Mysql_backend

However, I cannot figure out a way to tell Oasis to link the optional modules into the executable. I tried to figure out a way of doing this by modifying myocamlbuild.ml file but failed. Can I achieve this with the rule function described here?

If what I describe cannot be achieved with ocamlbuild, is there any ither tool that would do the job and avoid problems of OCamlMake?

Solution