Ruby offers [define_method](<http://ruby-doc.org/core-2.3.1/Module.html#method-i-define_method>) as a private method on modules and classes for defining new instance methods. However, the ‘body’ of the method must be a Proc or another existing method.

One way to create a method from raw string data is to use eval to create a Proc from the code:

xml = <<ENDXML
<methods>
  <method name="go">puts "I'm going!"</method>
  <method name="stop">7*6</method>
</methods>
ENDXML

class Foo
  def self.add_method(name,code)
    body = eval( "Proc.new{ #{code} }" )
    define_method(name,body)
  end
end

require 'nokogiri' # gem install nokogiri
doc = Nokogiri.XML(xml)
doc.xpath('//method').each do |meth|
  Foo.add_method( meth['name'], meth.text )
end

f = Foo.new
p Foo.instance_methods(false)  #=> [:go, :stop]
p f.public_methods(false)      #=> [:go, :stop]
f.go                           #=> "I'm going!"
p f.stop                       #=> 42