interface_for_foo.rb

class Foo
  def whatever
    puts 'whatever foo'
  end
end

module Fooish
  def whatever
    raise NotImplementedError, "This is a Fooish method, you need to implement the guts of it in #{self.is_a? Module ? self.name : self.class.name}."
  end
end

class Bar
  include Fooish
  
  def whatever
    puts 'whatever bar'
  end
end