Struct default values random behaviour when using env var

[jonhkr]

I have the following scenario where a struct is defined with default values that come from Application.get_env function:

defmodule M do
  defstruct a: Application.get_env(:app, :a), b: Application.get_env(:app, :b)
end

These env values come from the config.exs as follows:

config :app,
  a: "value for a",
  b: "value for b"

Sometimes when the application starts, the struct receives the values from the config, sometimes it just receives nil, actually, most of the time it receives nil.

I think that this happens because the configuration is being evaluated after the struct definition, then when it tries to get the values they weren't defined yet.

[josevalim]

This behavior is going to read the application configuration at compile
time. Is this what you really want?

José Valim
www.plataformatec.com.br
Skype: jv.ptec
Founder and Lead Developer

[jonhkr]

I don't know if I'm right, please correct me if not, I believe the struct default values are evaluated only once, when the struct is evaluated. In this case I think that the application configuration should be available so that the struct can receive its default values.

One use case for this is when I need to authenticate to an third party web service and want to have my authentication info in a struct without having to set it everytime. For instance, I can build the request like:

request = %Request{
  auth: %Auth{},
  foo: "bar"
}

Where Auth is defined like so:

defstruct key: Application.get_env(:app, :key), secret: Application.get_env(:app, :secret)

[ericmj]

Struct default values are evaluated only once - at compile time. If you have the key and secret when compiling you should be fine, if not you need to fetch it every time you create a struct.

[josevalim]

I could not reproduce this. The values are always there if they have been set at compilation time.

[jonhkr]

I'm sorry, I've not shown you the full scenario.

This behaviour happen when I have this struct in a project and I try to configure it from another project.

Say I have the project A that contains this struct that gets its default values from the application configuration. Then I create project B that depends on project A to work. Project B sets those environment variables which are not being read because project A is compiled before project B.

I think there should be a warning in the docs to tell people about this. It is documented here though, in case someone else runs in the same problem.

[josevalim]

Your approach is fine but you need to configure it before you add A as a dependency. If it is done afterwards, you should do mix deps.clean a so it gets compiled again. You should definitely add this information to your project readme because, I agree, it is not super obvious. :)