I think a great deal of the abuse of the application env occurs because it is simply easy to do so. It is more difficult to think about how to make your library or application configurable by feeding in parameters to a top-level supervisor or something, than it is to just stuff anything configurable into config.exs and use Application.get_env/3. As long as the easier path is available, there will always be people who take it. Erlang didn’t have quite as much of an issue with this, because it was more of a pain to use the application env for configuration than it was to configure via parameters.
I think a related issue is compile-time configuration. Some of the examples José mentions need configuration inputs at compile-time, and there is no other means to provide them except config.exs, since Mix evaluates it during compilation. Had Mix gone the route of Erlang in regards to application env, the whole discussion would be moot, since config.exs would be limited to runtime configuration. Since it did not, we also have the problem where config.exs has become the catch-all for anything configurable. People are using System.cmd/3 to get a git shorthash in config.exs, rather than doing something like -Dcommit_hash=$(git describe --tags --long) as a flag to the compiler, which is how such things are typically handled in just about every other language I’ve worked with. That isn’t suitable for everything though, and is unwieldy for repeated use. This is definitely a place where config.exs eases a lot of pain - compile-time parameters and global defaults.
I was hoping that by treating config.exs as a runtime config, and having mix.exs handle defining what to include or not include at compile-time, we could get the best of both worlds. It doesn’t really solve the issue of how to prevent people from abusing the app env, but I’m not sure that is something we can really fix at this point, about the only time you could make that kind of change is with a theoretical Elixir 2.0, but its not clear how you could do that without making configuration more inconvenient in general. It seems like a bit of a catch-22 to me.
