Example of using define_method in a mixin to define a method in an including class/module

define_method.rb

module MyMixin
  def self.included(other)
    other.instance_eval do
      define_method :bar do
        puts 'bar called'
      end
    end
  end
end

class MyClass
  include MyMixin
  def foo
    puts 'foo called'
  end
end

p MyClass.instance_methods(false)
# => [:bar, :foo]