About

Module attributes may be used like “constants” which are evaluated at compile-time.

defmodule Example do
  @number 2

  def number(), do: @number
end

However, they don’t strictly behave like constants because they can be overwritten by redefining them in the module:

defmodule Example do
  @standard_message "Hello, World!"
  @standard_message "Overwritten!"

  def message(), do: @standard_message
end

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Originally from Exercism elixir concepts