Metaprogramming

Metaprogramming
Alex Koppel
6Wunderkinder

Alex Koppel – 1/3/2012

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

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

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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

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Techniques

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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

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Code
require 'active_support/core_ext/string/inflections'!

# reopen String class!
class String!
# add a module!
include ActiveSupport::CoreExtensions::String::Inflections!
end!

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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

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class Foo!
Code
def bar!
3!
end!
# make a copy of bar!
alias_method :bar, :old_bar!
# redefine bar!
def bar!
4!
end!
end!

Foo.new.bar!
# => 4!
Foo.new.old_bar!
# => 3! 6Wunderkinder

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
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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] http://ruby-doc.org/docs/ProgrammingRuby/html/classes.html 6Wunderkinder

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

Code 2: Objects
str = "hello world"!
class << str!
def foo!
2!
end!
end!

str.foo!
# => 2!
"hello world".foo!
# => NoMethodError!

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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

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Code
def 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!
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How does it work?
! Get the individual string's 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

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Problem!
! Ruby's Marshal.dump is used to serialize objects

! Marshaling does NOT work with singletons
! Singletons can't be cached!

! Defining instance#_dump doesn't help
! Expects String#load to exist

! Use this carefully in shared code
! You never know what your users will do

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Solution!
! Use modules to mix in unique methods
! Ruby knows how to handle this, even for objects
module 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" !
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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
! Won't create the class if it doesn't exist

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Code
module 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!

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Method Missing
! Intercept calls to non-existent methods

! Make a class's interface dynamic

! Example: ActiveRecord
! Create dynamic finders on the fly
! @user.find_by_name_and_status

! Example: proxy objects

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Pseudo-code for
ActiveRecord 2.x
def 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 !
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Important Note
! method_missing can be much slower

! There are some tricky issues
! respond_to? and beyond
! Solving each problem can create others [1]

! Don't overuse it
! Can you use define_method instead?

[1] http://www.slideshare.net/paoloperrotta/the-revenge-of-methodmissing 6Wunderkinder

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
! Don't use these

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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

tl;dr
! Metaprogramming is awesome

! Don’t overuse it
! But don’t be afraid to use it when appropriate

! Enjoy :)

6Wunderkinder

Resources
! Ruby books

! URLs:
! http://yehudakatz.com/2009/11/15/metaprogramming-
in-ruby-its-all-about-the-self/
! http://ruby-doc.org/docs/ProgrammingRuby/html/
classes.html
! http://www.slideshare.net/paoloperrotta/the-
revenge-of-methodmissing
! http://pragprog.com/book/ppmetr/metaprogramming-
ruby

! Google "ruby metaprogramming“

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Questions?
@arsduo
@6Wunderkinder

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Metaprogramming

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