No longer generate config for mix new

[josevalim]

The fact we explicitly told developers to configure their
application in the config file and access it with
Application.get_env/2, probably led many to rely on
the application environment when better options are
available.

Furthermore, mix new is mostly used for libraries,
where config is even less important, as configuration
from libraries is not transitive.

Developers that need the config, can simply add it back
by calling:

echo "use Mix.Config" > config/config.exs

This commit removes config for new apps and deprecate
the --config flag, which cannot perform deep merges.
Note we do keep the config for umbrellas, as umbrellas
are about applications and not libraries (plus we need
the umbrella children to share and point to an existing
config).

Closes #8815.
Closes #8811.

[perrycate]

Pardon my ignorance, but what are the aforementioned "better options"? Based on this guide it seems the alternative to using a config.exs file is to add config parameters to env in application/0 in mix.exs.

There clearly are differences between the two, as I am unable to set module attributes using Application.fetch_env!/2 using the latter config option and using iex -S mix. I'm having trouble groking what those differences are, though, and what the best practices for configuration in general are.

(If it makes a difference, I have been learning elixir by working through the book "Programming Elixir >= 1.6 by Dave Thomas. It seems it was written before this decision was made, as the example I'm working through assumes config/config.exs exists and is the best place to put application configuration. Is there another resource that would be better for a new user such as myself to consult?)

[vans163]

Pardon my ignorance, but what are the aforementioned "better options"? Based on this guide it seems the alternative to using a config.exs file is to add config parameters to env in application/0 in mix.exs.

There clearly are differences between the two, as I am unable to set module attributes using Application.fetch_env!/2 using the latter config option and using iex -S mix. I'm having trouble groking what those differences are, though, and what the best practices for configuration in general are.

(If it makes a difference, I have been learning elixir by working through the book "Programming Elixir >= 1.6 by Dave Thomas. It seems it was written before this decision was made, as the example I'm working through assumes config/config.exs exists and is the best place to put application configuration. Is there another resource that would be better for a new user such as myself to consult?)

An option is to:
Create a .env file and source it or pass env vars directly on commandline. Then use :persistent_term.put/2, :persistent_term.get/1 to set it emulator wide in your initial module once you read it back from env.