mattr_reader

Defines a class attribute and creates a class and instance reader methods. The underlying the class variable is set to nil, if it is not previously defined.

module HairColors mattr_reader :hair_colors end HairColors.hair_colors # => nil HairColors.class_variable_set("@@hair_colors", [:brown, :black]) HairColors.hair_colors # => [:brown, :black] 

The attribute name must be a valid method name in Ruby.

module Foo mattr_reader :"1_Badname " end # => NameError: invalid attribute name 

If you want to opt out the creation on the instance reader method, pass instance_reader: false or instance_accessor: false.

module HairColors mattr_writer :hair_colors, instance_reader: false end class Person include HairColors end Person.new.hair_colors # => NoMethodError 

Also, you can pass a block to set up the attribute with a default value.

module HairColors cattr_reader :hair_colors do [:brown, :black, :blonde, :red] end end class Person include HairColors end Person.hair_colors # => [:brown, :black, :blonde, :red]