Can "Application.get_env(..)" be used as a module attribute along with "runtime.exs"?

Questions & Help Questions
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1

Somewhat a legacy project. There’re Application.get_env(...) and Application.get_env(...)[:some_key] as the module attributes here and there. I’ve replaced releases.exs with runtime.exs.

Yet I’m confused: will Application.get_env(...) as the module attributes work properly then? Will they point to the compile time config values or the runtime ones?

2

Compile time. Module attributes are compile time constructs and don’t actually exist at runtime.

3

You could replace those module attributes by simple functions to have runtime evalution… and benefit from runtime.exs

4

Additionally the same already applied with config/releases.exs.

5

What do you mean? What “the same”, “applied” what?

6

config/releases.exs also ran after your code was compiled, so it also couldn‘t change any config you compiled into module attributes.

7

Another option is to use Application.compile_env!/3 if you want to keep the module attributes with compile time config values instead of replacing them with runtime evaluated functions.

8

Is there a way to use both - compile-time and runtime values - such that the runtime ones will have the preferance? If a runtime value has been provided, use it, otherwise - fallback to the one specified statically, in config/{dev,prod,qa,test,etc}.exs

9

The following should work, but I would not encourage it. Now your code is linked to 2 configs and in the future it’s hard to know which env returns the value.

def get_value() do
  Application.get_env(runtime_value...) || Application.compile_env!(compiletime_value...)
end
10

Just define the value in both config/{dev,test,prod}.exs and config/runtime.exs. Runtime will win if the key is the same. Consider:

If the key/value exists only in compile time config:

# config/dev.exs
config :my_app, :some_key, "Compile time!"

Application.get_env(:my_app, :some_key) => "Compile time!"

If you define it in both:

# config/dev.exs
config :my_app, :some_key, "Compile time!"

# config/runtime.exs
config :my_app, :some_key, "Runtime!"

and then in IEx:

Application.get_env(:my_app, :some_key) => "Runtime!"

That said, you cannot expect the module attributes to respect this, because they are generated at compile time and the runtime config does not yet exist.

I would switch to functions to save on disk and memory usage, anyways. See this from Sara Juric about how module attributes as constants have a potential gotcha: Blog Post: 10 Elixir gotchas - #16 by sasajuric

11

And one step futher: if the same section contains some keys-values as compile-time ones, others as runtime ones?

# config/dev.exs
config :my_app, MyModule, 
  k1: "Compile time!"
  k2: "compile time2!"

and

# config/runtime.exs
config :my_app, MyModule, 
  k2: "runtime time2!"
  k3: "runtime time3!"

Will Application.get_env(:my_app, :MyModule)[:kN] work as intended always?

12

Yes.

You can think of Config values as sort of a Map.merge. The last defined value always wins (runtime.exs by default).

In your example, with both of those defined in config/dev.exs and config/runtime.exs, you will get:

iex(1)> Application.get_env(:my_app, MyModule)
[k1: "Compile time!", k2: "runtime time2!", k3: "runtime time3!"]
13

What to resolve this issue

a) I have to use a module attribute – in order to embed it into other modules via a macro.
b) Yet I also want to be able to pass a variable that a said module attribute reads from config dynamically, at runtime.

?

defmodule MyMod1 do
  defmacro __using__(_) do
    quote do

      # @a1 Application.compile_env(:my_app, My.App)[:key1]
      @a1 Application.get_env(:my_app, My.App)[:key1]

      # ............

Only by replacing an attibute with a function?

14

For someone writing custom macros this is sure a strange question.

Yes, module attributes are always evaluated at compile-time, either use a function or an anonymous function in your attribute.