Elixir: Attributes

Sometimes, in a module, you need to put some values into constants, for this use attributes starting with @ sign.

defmodule MyModule do
  @magic_number 8

  def do_magic(num) do
    num * @magic_number
  end
end

MyModule.do_magic(10) # 80

Module attributes are not available from outside the module.

defmodule MyModule do
  @magic_number 8
end

MyModule.@magic_number # raises error

Attributes can be declared several times and redefined, but it should be noted that the compiler inlines the last value of the declared attribute, for example:

defmodule MyModule do
  @magic_number 8

  def cast_magic() do
    @magic_number
  end

  @magic_number 0

  def do_magic() do
    @magic_number
  end
end

MyModule.cast_magic() # 8

MyModule.do_magic() # 0

# after compilation module attributes inlines like this 
defmodule MyModule do
  @magic_number 8

  def cast_magic() do
    8
  end

  @magic_number 0

  def do_magic() do
    0
  end
end

Within modules, there are also special attributes that are used by Elixir to generate documentation, such as the @moduledoc attribute that describes general information about the module or the @doc attribute that documents a declared function:

defmodule MyModule do
  @moduledoc "My attributes exercise module."

  @magic_number 8

  @doc "Do some magic calculations."
  def do_magic(num) do
    num * @magic_number
  end
end

Then, these attributes are used in documentation generation.

You can also make a module attribute accumulate redefined values, with a special attribute declaration, for example:

defmodule MyModule do
 Module.register_attribute __MODULE__, :magic_values, accumulate: true

 @magic_values 8
 @magic_values :some
 @magic_values "hello"

 def do_magic() do
   @magic_values
 end
end

MyModule.do_magic # [8, :some, "hello"]

Instructions

Define the following attributes with values inside the module:

• Attribute: number_attr, value: 10
• Attribute: boolean_attr, value: false
• Attribute: hello_world_attr, value: "Hello, World!"