Metaprogramming Ruby 2 Paolo Perrotta

Spell is notation of a usefull patterns that you can use.

Metaprogramming is used to create a wrap around external system, for example call any method which will be dynamically called on the system (no need to define methods). You can mimics specific language which is more suitable for solving that particular problem. Also you can add logging functionality around method that you want to monitor, run custom code whenever a client inherits from your class…

In C++ (not interpreted language) at runtime (after compilation finishes) you can not ask for class methods, but in Ruby, you can use introspection for example my_object.class.

Metaprogramming is writting code that manipulates language constructs at runtime.

Instead of calling functional style to_alphanumeric('string') you can reopen String class and use like 'string'.to_alphanumeric. This is called Monkeypatching.

Spell Refinement

You can refine classes inside module so you do not monkeypatch global class

module StringExtensions
  refine String do
    def to_alphanumeric
      gsub(/[^\w\s]/, '')
    end
  end
end

"a*".to_alphanumeric # NoMethodError
using StringExtensions
"a*".to_alphanumeric # 'a'

module StringStuff
  using StringExtensions # this is active only till the end of module
  "a*".to_alphanumeric # 'a'
end

Even object.instance_variables live in the object itself, its object.methods is actually using class instance methods object.class.instance_methods. Each class is also an object and as an object, it contains instance methods

Class.instance_methods(false) # => [:new, :allocate, :superclass]

You can use .superclass method to find a chain

Array.superclass # => Object
Object.superclass # => BasicObject
BasicObject.superclass # => nil

For Class.superclass == Module so class and module is almost the same, except classes can call .new so we use them to instantiate, and we use module to include it somewhere (since Class is a subclass of Module it can be used as module, but we keep using only where we instantiate them). Using class MyClass you are actually defining (or reopening) a instance of Class and assigning to constant MyClass. Since MyClass is an object and class in the same time, we can call .class (right ->) and .superclass (up ^) Note that MyClass.class == Class and MyClass.superclass == Object. Also Class.superclass == Module and Module.superclass == Object so all classes are objects and they inherit from Object class. Object.class == Class, Module.class == Class and Class.class == Class.

Constant differs from variables in that way you can access them in any scope

M::C::MyConst
# `Module` class provides instance method and class method `constants`.
M.constants # => [:C]
Module.constants # => all global constants

Method lookup: to find a method, ruby goes in the receiver’s class (class of the object on which you call the method) and than climbs the ancestors chain. Anchestors chain also include modules that are prepended or included in class.

Spell KernelMethod

You can add method to Kernel so you can use it in every object, like awesome print ap 'string'

Every line of ruby code is executed inside an object, current object or self.

In static type checking, whether method exists is checked at compile time. It is an advantage, but the price for that is that you have to write a lot of boilerplate methods like getters, setters, and delegate methods.

Spell Dynamic Dispatch

Using send like my_object.send :my_method, 123 you can decide which method to call at runntime. With .send you can call private methods (use .public_send to prevent calling private methods).

Spell Dynamic Method

You can define methods on the fly using Module#define_method

class MyClass
  define_method :my_method do |my_arg|
    my_arg * 3
  end

  # or use it inside class method
  def self.define_component(name)
    define_method(name) do
    end
  end
end

If you need to define method name for each associated object method, you can iterate over associated_object.methods.grep(/^get_(.*)/) { MyClass.define_component $1 }.

Spell Ghost Method

You can override Object#method_missing so you can respond to any method

class MyClass
  def method_missing(method_name, *args, &blk)
  end
end

Spell Dynamic Proxy

Object that catches Ghost Methods and forwards them to another object is called dynamic proxy.

class MyClass
  class ResourceProxy
    def method_missing(message, *args, &block)
      subject.send(message, *args, &block)
    end

    def subject
      @subject ||= connection.get(path_prefix).body
    end
  end
end

class MyClass
  module API
    module Gists
      class Proxy < ::MyClass::ResourceProxy
        def star
          connection.put("#{path_prefix}/star").status == 204
        end
      end
    end
  end
end

# when you call gist#star it will call put request, but for other
# like, gist.description it proxy it to subject which will read description
# attribute from response (using Hashie)

If you call .respond_to? :dynamic_method than you will get false, so when overriding method_missing we should also override respond_to_missing? Note that you should call super for method names that you do not proccess, otherwise you can get stack level too deep because of infinite loop when some undefined method is called. Because of this it is prefered to use Dynamic Methods instead Ghost Methods.

class MyClass
  def method_missing(name)
    super if !@data_source.respond_to?("get_#{name}_info")

    info = @data_source.send("get_#{name}_info", @id)
    price = @data_source.send("get_#{name}_price", @id)
    result = "#{name.capitalize}: #{info} ($#{price})"
    return "* #{result}" if price >= 100
    result
  end

  def respond_to_missing?(method, include_private = false)
    @data_source.respond_to?("get_#{method}_info") || super
  end
end

Skill Blank Slate

When the name of method could be inherited from Object (like method .display) than we can not use it (since it is not method missing), there are two solutions: inherit from BasicObject (not implicit Object) or removing methods with #undef_method or #remove_method for each instance_methods.

class MyClass < BasicObject
  # no need respond_to_missing since BasicObject.instance_methods are few
  # => [:__send__, :!, :==, :!=, :equal?, :__id__, :instance_eval, :instance_exec]
end

# or undefining all except __methods
class MyClass
  def self.hide(name)
    if name !~ /^(__|instance_eval$)/
      undef_method name
    end
  end

  instance_methods.each { |m| hide(m) }
end

You can also use .const_missing as in Rake

class Module
  def const_missing(const_name)
    case const_name
    when :Task
      Rake.application.const_warning(const_name)
      Rake::Task
    when :FileTask
      Rake.application.const_warning(const_name)
      Rake::FileTask
    when :FileCreationTask
      # ...
    end
  end
end

Blocks are defined only when you call a method. Inside method you can call yield to evaluate that block. You can check if block is given with block_given?. Using yield without params will call the block using the same params as method. Here is example to always call resource.dispose (when exception occurs or without exception in block).

module Kernel
  def using(resource)
    begin
      yield
    ensure
      resource.dispose
    end
  end
end

use(r) { |r| r.read }

When you create a block, it captures local bindings at that moment into a closure. When you yield, it will use that scope (not the scope of method where yield is called). Use Kernel#local_variables to list all current object local variables.

Spell Scope Gate

Scope changes when program enters a class or module definition or enters the method def (method definition is executed when it is called).

my_var = "Success"
class MyClass
  # We want to print my_var here...
  def my_method
    # ..and here
  end
end

Spell Flat Scope

To pass local variable through class gate, you can use Class.new block and define_method name, block to create nested lexical scopes (flattening the scope). You can inherit from existing class using Class.new(MyParrent) do.

my_var = "Success"
MyClass = Class.new do
  "#{my_var} in the class definition"

  define_method :my_method do
    "#{my_var} in the method"
  end
end

Spell Shared Scope

Control the sharing of variables by using Dynamic Dispatch inside the same flat scope.

def define_methods
  shared = 0

  Module.send :define_method, :counter do
    shared
  end
  Module.send :define_method, :inc do |x|
    shared += x
  end
end

Spell Context Probe

Use object.instance_eval block to evaluate block with the object as self. It’s like a snippet of code that you dip inside an object.

v = 2
obj.instance_eval { @v = v}
obj.instance_eval { @v} # => 2

There is also Module#class_eval (we are not changing only self but also current_class). It is more flexible than using class MyClass because we do not open new scope, we keep current bindings like Spell Flat Scope.

a_class.class_eval do
  # define instance_method
  def m; 'Hi'; end
end

Spell Clean Room

Create an object just to evaluate blocks inside it (in clear env, otherwise blocks could clash with current env)

class CleanRoom
  def current_temperature
    # ...
  end
end
clean_room = CleanRoom.new
clean_room.instance_eval do
  if current_temperature < 20
    # TODO: wear jacket
  end
end

Spell Deferred Evaluation

Use obj = Proc.new block to create object that holds a block and later use it with obj.call 123.

inc = Proc.new {|x| x + 1}
inc.call 2

Spell Class Instance Variable

Define instance variable in context of class object. Using double @@my_var is class variable which can be accessed from subclasses and instance methods.

class MyClass
  @my_class_var = 1
  def m
    # we can not access @my_class_var
  end
end

Spell Singleton Method

Define method for one specific object

str = 'Hi'

def str.title?
  self.upcase == self
end
# or
str.define_singleton_method(:title?){ self.upcase == self }

Spell Class Macro

Module#attr_accessor class method is called Class macro. They looks like keywords, but there are just regular class methods.

class MyClass
  attr_accessor :my_attribute

  # it will actually do something like
  def self.attr_accessor(name)
    define_method "#{name}=" do |value|
      send "@#{name}=", value
    end
  end
end

It can be used for deprecation warnings

class Book
  def title
  end

  def self.deprecate(old_method, new_method)
    define_method(old_method) do |*args, &block|
      warn "Warning: #{old_method}() is deprecated. Use #{new_method}()."
      send(new_method, *args, &block)
    end
  end

  deprecate :GetTitle, :title
end

Singleton class of the object (metaclass, eigenclass) ie each object can have it’s own special hidden class. Special syntax needs to be used class << object and you can get it with object.singleton_class. It is a place where singleton methods live. It’s superclass is object class.

obj = My.new
def obj.my_singleton_method; end

# similar but it opens new scope definee cref
class << obj
  def my_singleton_method
  end
end

obj.singleton_class.instance_methods.grep /my_/
obj.singleton_class.superclass == My

So method lookup also includes singleton class, let’s call it #obj class.

We can also add class method using same notation

class C
  class << self
    def a_class_method
      'C.a_class_method'
    end
  end

  # same as
  # def self.a_class_method
  #   'C.a_class_method'
  # end
end
# or third way of defining class methods is outside of class
# def C.a_class_method

class D < C
end

D.a_class_method # => 'C.a_class_method()'

So method lookup for D.a_class_method also looks at singleton class of superclass #C. Seven rules or ruby object model

• there is only one kind of object: regular or a module
• there is only one kind of module: regular module, class or singleton class
• there is only one kind of method and it lives in a module (most often in a class)
• every object (classes included), has its own ‘real class’, be it a regular class or a singleton class
• every class, except BasicObject, has exactly one ancestor, either a superclass or a module.
• superclass of singleton class of an object is the object’s class. The superclass of singleton class of a class is the singleton class of the class’s superclass
• when you call a method, ruby goes right to the receiver’s real class and than ‘up’ the ancestors chain.

Spell Class Extension

When you want to include module methods as class methods

module MyModule
  def my_method; 'hello'; end
end

class MyClass
  class << self
    include MyModule
  end
end

# short way is using
class MyClass
  extend MyModule
end

Spell Object Extension

You can extend objects with methods

module MyModule
  def my_method; 'hello'; end
end

obj = Object.new

class << obj
  include MyModule
end

obj.my_method # => 'hello'

# another way is to use
obj.extend MyModule

Spell Around Alias

Three steps: alias the method, redefine original method, call old method from new method

module Kernel
  alias_method :new_method, :original_method

  def original_method(args)
    # add code here
    new_method(args)
  end
end

for example

class String
  alias_method :real_length, :length

  def length
    real_length > 5 ? 'long' : 'short'
  end
end

Spell Refinement Wrapper

Using super inside refinement will call original method

module StringRefinement
  refine String do
    def length
      super > 5 ? 'long' : 'short'
    end
  end
end

Spell Prepended Wrapper

Using prepend (which is searched before class method, or included methods) so also you can use super to call original method. It is much cleaner than spell around alias (which looks like Monkey patching).

module ExplicitString
  def length
    super > 5 ? 'long' : 'short'
  end
end

String.class_eval do
  prepend ExplicitString
end

So you can use this to wrap some library methods so you can catch exceptions from it

module AmazonWrapper
  def reviews_of(book)
    result = super
    result
  rescue
    []
  end
end

Amazon.class_eval do
  prepend AmazonWrapper
end

Spell String of Code

Kernel#eval can run string of code.

array = [1]
el = 2
eval "array << el"

# or define methods which also depend on verb
POSSIBLE_VERBS = ['get', 'put', 'post', 'delete']
POSSIBLE_VERBS.each do |m|
  eval <<-end_eval
    def #{m}(path, *args, &b)
      r[path].#{m}(*args, &b)
    end
  end_eval
end

It is used with binding so you can evaluate code in some context (similar to closure but it does not contains code just scope). In ruby there is TOPLEVEL_BINDING.

Spell Code Processor

eval statements, @bindings, file, line

Instead eval string you can use obj.instance_eval which can take string or block (both can access local variables).

array = ['a', 'b', 'c']
x = 'd'
array.instance_eval "self[1] = x"

Spell Hook Method

You can override Class#included to provide a hook into particular event (inherited, included, prepended, extended, also method_added, method_removed and method_undefined)

class String
  def self.included(subclass)
    puts "#{self} was included by #{subclass}"
  end
end

class MyString < String; end

This can be used to include MyModule which will extend methods so you got class macros which you can use. This is also called include and extend trick.

module CheckedAttributes
  def self.included(base)
    base.extend ClassMethods
  end

  module ClassMethods
    def my_class_macro(name, &block)
    ...

class MyClass
  include CheckedAttributes

Instead of overriding self.included you can override include and use super but it is not so clear since you have to call super

class C
  def self.include(*modules)
    puts "Called: C.include(#{modules})"
    super
  end

  include M
end

Problem with extending is with chained extensions, since at the momen first module include SecondLevel module, it’s def self.included(base) base will point to first level module, not the target class.