Eliminate need for class eval

activerecord.rb

# lib/carrierwave/orm/activerecord.rb from jnicklas/carrierwave


# before
class_eval <<-RUBY, __FILE__, __LINE__+1
  def #{column}=(new_file)
    column = _mounter(:#{column}).serialization_column
    send(:"\#{column}_will_change!")
    super
  end

  def remote_#{column}_url=(url)
    column = _mounter(:#{column}).serialization_column
    send(:"\#{column}_will_change!")
    super
  end

  def remove_#{column}!
    super
    _mounter(:#{column}).remove = true
    _mounter(:#{column}).write_identifier
  end

  def serializable_hash(options=nil)
    hash = {}

    except = options && options[:except] && Array.wrap(options[:except]).map(&:to_s)
    only   = options && options[:only]   && Array.wrap(options[:only]).map(&:to_s)

    self.class.uploaders.each do |column, uploader|
      if (!only && !except) || (only && only.include?(column.to_s)) || (except && !except.include?(column.to_s))
        hash[column.to_s] = _mounter(column).uploader.serializable_hash
      end
    end
    super(options).merge(hash)
  end
RUBY

# after
define_method("#{column}=") do |new_file|
  serialization_column = _mounter(column.to_sym).serialization_column
  send(:"#{serialization_column}_will_change!")
  super
end

define_method("remote_#{column}_url=") do |url|
  serialization_column = _mounter(column.to_sym).serialization_column
  send(:"#{serialization_column}_will_change!")
  super
end

define_method("remove_#{column}!") do
  super
  _mounter(column.to_sym).remove = true
  _mounter(column.to_sym).write_identifier
end

def serializable_hash(options = nil)
  hash = {}

  except = options && options[:except] && Array.wrap(options[:except]).map(&:to_s)
  only   = options && options[:only]   && Array.wrap(options[:only]).map(&:to_s)

  self.class.uploaders.each do |column, uploader|
    if (!only && !except) || (only && only.include?(column.to_s)) || (except && !except.include?(column.to_s))
      hash[column.to_s] = _mounter(column).uploader.serializable_hash
    end
  end
  super(options).merge(hash)
end

connection.rb

# lib/faraday/connection.rb from technoweenie/faraday

# before
%w[get head delete].each do |method|
  class_eval <<-RUBY, __FILE__, __LINE__ + 1
    def #{method}(url = nil, params = nil, headers = nil)
      run_request(:#{method}, url, nil, headers) { |request|
        request.params.update(params) if params
        yield request if block_given?
      }
    end
  RUBY
end

# after
[:get, :head, :delete].each do |method|
  define_method(method) do |url = nil, params = nil, headers = nil|
    run_request(method, url, nil, headers) { |request|
      request.params.update(params) if params
      yield request if block_given?
    }
  end
end

Some things that are wrong with your refactoring:

1. define_method and methods defined with Ruby's def syntax are not interchangeable. The former creates a closure, which is something that you might want in some cases, but want to avoid in other cases. With def, you're sure that the method has its own scope.

2. You can't call super without brackets inside a method created with define_method, otherwise you will get:

   RuntimeError: implicit argument passing of super from method defined by define_method() is not supported.
   Specify all arguments explicitly
   

3. You can't do define_method(method) do |url = nil| (have a default value) in Ruby 1.8

4. Methods created with define_method are slower to call. Observe:

   require 'benchmark'
   
   dummy = Class.new {
     def norm() end
     define_method(:meta) {}
   }.new
   
   n = 5000000
   Benchmark.bm do |x|
     x.report('norm') { n.times { dummy.norm } }
     x.report('meta') { n.times { dummy.meta } }
   end

Thank you, I really didn't know about most of those things. Now it's much clearer why do many people use it.

But I don't think this benchmark is significant, because then there would be a downsize in refactoring a big method into many smaller ones.

Also, by "ugly" I obviously meant class_eval, not define_method :P

http://tenderlovemaking.com/2013/03/03/dynamic_method_definitions.html