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A review of basic and advanced metaprogramming techniques in Ruby, with examples of both the pitfalls and the workarounds you may encounter.

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  1. 1. MetaprogrammingAlex Koppel6Wunderkinder Alex Koppel – 1/3/2012
  2. 2. What is metaprogramming?!   "Like programming, only meta" – an old colleague!   Code that writes code!   "Metaprogramming is the writing of computer programs that write or manipulate other programs (or themselves) as their data, or that do part of the work at run time that would otherwise be done at compile time.“ – Wikipedia Alex Koppel - 6Wunderkinder
  3. 3. Why should you care?!   Ruby allows metaprogramming!   Metaprogramming can create stunningly effective tools !   Rails is metaprogramming!   You want to be a more effective Ruby programmer 6Wunderkinder
  4. 4. Caution!!   Wield metaprogramming deliberately!   Your code will be harder to understand !   Will other developers lose time figuring out your magic?!   Your code will be more brittle !   Altering other classes = dependency !   Changes to underlying code can break your app !   c.f. Facebooker 6Wunderkinder
  5. 5. Techniques 6Wunderkinder
  6. 6. Reopening Classes!   Add additional methods or variables to existing classes!   Very simple, very powerful!   New code is executed as your files are loaded !   In a sense, this is a "static" metaprogramming !   Common in monkeypatching!   Caveat: this creates the class if not yet defined 6Wunderkinder
  7. 7. Coderequire active_support/core_ext/string/inflections!# reopen String class!class String! # add a module! include ActiveSupport::CoreExtensions::String::Inflections!end! 6Wunderkinder
  8. 8. alias_method!   alias_method(new_name, old_name)!   Makes an copy of a method !   exist v. exists!   Copy remains if the original is overridden !   You may remember alias_method_chain!   Can be used dynamically in other methods 6Wunderkinder
  9. 9. class Foo! Code def bar! 3! end! # make a copy of bar! alias_method :bar, :old_bar! # redefine bar! def bar! 4! end!end!!# => 4!!# => 3! 6Wunderkinder
  10. 10. Metaclasses!   All Ruby objects have a unique class !   The "metaclass", a.k.a. "eigenclass" !   Classes too !   Special cases: Fixnums, true, false, nil!   You can access this class to change the object !   class << object opens up the metaclass !   def object.new_method and object.instance_eval also work!   Note: reopening classes and alias_method alter the original class, not a metaclass 6Wunderkinder
  11. 11. What are Objects and Classes, really? !   An object is actually simple[1] !   A collection of instance variables !   A reference to a class !   When you call a method, Ruby goes up the class hierarchy for its definition !   Metaclass, then class, then superclass !   Changing methods on an individual object is really changing its metaclass[1] 6Wunderkinder
  12. 12. Code 1: Classes# Using class << self for a class is effectively!# equivalent to reopening the class (see above)!class Person!! # this can be easier than writing tons of def self.s ! class << self! def species! "Homo Sapien"! end! end!end!class << Person! def species; "Homo Sapien“; end!end!Person.species == “Homo Sapien“ # true! 6Wunderkinder
  13. 13. Code 2: Objectsstr = "hello world"!class << str! def foo! 2! end!end!!# => 2!"hello world".foo!# => NoMethodError! 6Wunderkinder
  14. 14. Case study: non-breaking changes!   Example from Koala (Facebook SDK)!   Changing a method to return strings rather than hashes!   Could this be done without breaking apps?!   Use metaprogramming on the returned strings !   Add missing methods !   Print deprecation warning 6Wunderkinder
  15. 15. Codedef get_app_access_token! info = get_app_access_token_info # hash! string = info["access_token"]! # make the string compatible with hash accessors! command = <<-EOS! def [](index)! puts "DEPRECATION WARNING"! hash = #{hash.inspect} ! hash[index]! end! EOS! # add those methods just to the string! (class << string; self; end).class_eval command!end! 6Wunderkinder
  16. 16. How does it work?!   Get the individual strings metaclass: !   (class << str; self; end) !   Opens the class scope, and returns the (meta)class!   class_eval !   Executes code in the scope of a class (or metaclass) !   class_eval requires a class, not an instance !   String eval can be used for dynamic content !   You can also eval a regular code block!   Objects altered like this are "singletons" !   Has runtime implications 6Wunderkinder
  17. 17. Problem!!   Rubys Marshal.dump is used to serialize objects!   Marshaling does NOT work with singletons !   Singletons cant be cached!!   Defining instance#_dump doesnt help !   Expects String#load to exist!   Use this carefully in shared code !   You never know what your users will do 6Wunderkinder
  18. 18. Solution!!   Use modules to mix in unique methods !   Ruby knows how to handle this, even for objectsmodule BackwardsCompatible! def [](key)! puts "Got key! #{key}"! end!end!my_string = "abc”!# objects:extend <=> classes:include !my_string.send(:extend, BackwardsCompatible)!my_string["foo"]!Got key! foo! => nil !Marshal.dump(my_string)! => "x04bIe:x18BackwardsCompatible"babcx06:x06ET" ! 6Wunderkinder
  19. 19. class_eval again!   Ruby pattern: modules updating the including class !   Add before_filters, attrs, etc. !   Could use .send in some cases !   class_eval is easier to read !   Can define dynamic methods !   ActiveSupport::Concern works this way!   class_eval is like reopening a class !   But can be used without knowing the class in advance !   Wont create the class if it doesnt exist 6Wunderkinder
  20. 20. Codemodule ControllerStuff! def self.included(base)! # execute new code in the scope of! # the including class! base.class_eval do! before_filter :module_method! attr_accessor :module_var! @module_var = 2! end! end!end! 6Wunderkinder
  21. 21. Method Missing!   Intercept calls to non-existent methods!   Make a classs interface dynamic!   Example: ActiveRecord !   Create dynamic finders on the fly !   @user.find_by_name_and_status!   Example: proxy objects 6Wunderkinder
  22. 22. Pseudo-code for ActiveRecord 2.xdef method_missing(method_id, *args, &block)! # is it the right find_by_...format?!! if match(method_id) && match.finder?!! # method_missing is a class method ! self.class_eval %{ # string! # define the method! def self.#{method_id}(*args)! find(:#{finder}, options.merge ! (finder_options))! end! end! # now call the new method! send(method_id, *arguments)! end!end ! 6Wunderkinder
  23. 23. Important Note !   method_missing can be much slower !   There are some tricky issues !   respond_to? and beyond !   Solving each problem can create others [1] !   Dont overuse it !   Can you use define_method instead?[1] 6Wunderkinder
  24. 24. Reaching into Objects!   send: call any method on an object (public or not) !   Useful for dynamic programming !   Can also be used to break encapsulation !   public_send (1.9) is safer!   instance_variable_get, instance_variable_set: !   Access variables that lack an accessor !   Dont use these 6Wunderkinder
  25. 25. Code# from Koala (older)!# Scope: class with Typhoeous module included!def self.make_request(path, args, verb)! # dynamically call the appropriate method! # corresponding to the verb requested !! self.send(verb, url, :params => args)!End!# from Rails!# Scope: a helper class that needs a !# private controller method!def universal_calculation(arg)! # call the method on the controller! controller.send(:universal_controller, arg)!end ! 6Wunderkinder
  26. 26. tl;dr!   Metaprogramming is awesome!   Don’t overuse it !   But don’t be afraid to use it when appropriate!   Enjoy :) 6Wunderkinder
  27. 27. Resources!   Ruby books!   URLs: ! in-ruby-its-all-about-the-self/ ! classes.html ! revenge-of-methodmissing ! ruby!   Google "ruby metaprogramming“ 6Wunderkinder
  28. 28. Questions? @arsduo@6Wunderkinder 6Wunderkinder