Module attribute constants nil in Elixir

Module attributes in elixir (@something) can be used as constants and assigned a value, however one must make sure that the value assigned to the constant is available at compile time.

For example when using a constant that loads environment variables if the value is not available in compile time they will resolve to nil.

@salt System.get_env(:your_app_name, "AUTH_SALT")

So either export the environment variables before compiling, or don't use module attributes, instead you can use a function:

defp salt do
  System.get_env(:your_app_name, "AUTH_SALT")
end

Consider:

• The downside to using a function is that it will be re-evaluated each time.
• The downside of exporting env-vars before compile is that they you might forget and your app will crash in production. You can circumvent that by writing a script for compilation.
• You can also call System.get_env from your config.exs but make sure to run mix clean after doing so since the compiler seems to cache the config compilation.