Examples of Metaprogramming in Ruby

Define a method if it's not already defined

Code could be found in the i18n gem.

https://github.com/svenfuchs/i18n/blob/master/lib/i18n/core_ext/hash.rb

class Hash

  …

  def deep_merge!(data)
    merge!(data, &MERGER)
  end unless Hash.method_defined?(:deep_merge!)

  …

end

By adding unless Hash.method_defined?(:deep_merge!) after the end of the method, Hash.deep_merge! will be defined if it's not already defined.
This prevents monkey patch.
In Rails, active_support also defines Hash.deep_merge!. So i18n doesn't override that method.