class Module

Person.class_variable_get("@@hair_colors") # => [:brown, :black, :blonde, :red]

end
include HairColors
class Person

end
mattr_accessor :hair_colors, default: [:brown, :black, :blonde, :red]
module HairColors

You can set a default value for the attribute.

Person.new.hair_colors # => NoMethodError
Person.new.hair_colors = [:brown] # => NoMethodError

end
include HairColors
class Person

end
mattr_accessor :hair_colors, instance_accessor: false
module HairColors

Or pass instance_accessor: false, to omit both instance methods.

Person.new.hair_colors # => NoMethodError
Person.new.hair_colors = [:brown] # => NoMethodError

end
include HairColors
class Person

end
mattr_accessor :hair_colors, instance_writer: false, instance_reader: false
module HairColors

To omit the instance reader method, pass instance_reader: false.
To omit the instance writer method, pass instance_writer: false.

Person.new.hair_colors # => [:brown, :black, :blonde, :red, :blue]
Citizen.new.hair_colors << :blue

end
class Citizen < Person

change the value of subclasses too.
parent class. Similarly if parent class changes the value then that would
If a subclass changes the value then that would also change the value for

Person.new.hair_colors # => [:brown, :black, :blonde, :red]
HairColors.hair_colors # => [:brown, :black, :blonde, :red]
HairColors.hair_colors = [:brown, :black, :blonde, :red]

end
include HairColors
class Person

end
mattr_accessor :hair_colors
module HairColors

this method is called with a private or protected access modifier.
All class and instance methods created will be public, even if
Defines both class and instance accessors for class attributes.

def mattr_accessor(*syms, instance_reader: true, instance_writer: true, instance_accessor: true, default: nil, &blk)
  location = caller_locations(1, 1).first
  mattr_reader(*syms, instance_reader: instance_reader, instance_accessor: instance_accessor, default: default, location: location, &blk)
  mattr_writer(*syms, instance_writer: instance_writer, instance_accessor: instance_accessor, default: default, location: location)
end