Module Attribute behaviour during compilation

This is correct. Module attributes always have the value assigned to it in lines above the call. Basically you “hardcode” the parameter of IO.inspect at the time line 5 is compiled.

20 isn’t in the list, because you only add it to the list after reading it. Each usage of a module attribute will only ever know about values that have been assigned before its usage, pretty much like regular variables.

@LostKobrakai and obbZ thanks for your quick response. I understand now.

One quick question about Module.register_attribute/3, is this function used to track the changes made to a module attribute?

For note, module attributes are a lot like normal bindings, just saving you from typing the unquote part.

So this:

defmodule ModuleAttribute do
  @thing 1
  @thing 2
  def blah(), do: @thing # returns 2
  @thing 3
end

Is the same as this:

defmodule ModuleAttribute do
  thing = 1
  thing = 2
  def blah(), do: unquote(thing) # returns 2
  thing = 3
end

And thus this with an accumulated attribute:

defmodule ModuleAttribute do
  Module.register_attribute(__MODULE__, :thing, accumulate: true)
  @thing 1
  @thing 2
  def blah(), do: @thing # returns [2, 1]
  @thing 3
end

Is the same as this:

defmodule ModuleAttribute do
  thing = []
  thing = [1 | thing]
  thing = [2 | thing] # So this is now [2, 1]
  def blah(), do: unquote(thing) # returns [2, 1]
  thing = [3 | thing]
end