mattr_reader

Defines a class attribute and creates a class and instance reader methods. The underlying class variable is set to nil, if it is not previously defined. All class and instance methods created will be public, even if this method is called with a private or protected access modifier.

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: 1_Badname 

To omit the instance reader method, pass instance_reader: false or instance_accessor: false.

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

You can set a default value for the attribute.

module HairColors mattr_reader :hair_colors, default: [:brown, :black, :blonde, :red] end class Person include HairColors end Person.new.hair_colors # => [:brown, :black, :blonde, :red]