Ruby Metaprogramming Tutorial Part 1 - Introduction and Background

2017-03-23

Introduction

Ruby provides a convenient syntax for metaprogramming but it is often unclear when other libraries are using metaprogramming

Ruby is reflective, you can take a look at the properties of all objects.

Ruby’s facilities for DSL authoring don’t change the nature of the language. Ruby’s metaprogramming facilities are inherently tied to Ruby syntax and semantics, and whatever you write has to be shunted into Ruby’s object model.

The often repeated Ruby mantra is “Don’t Repeat Yourself” (DRY). In Ruby this is mainly achieved through the use of Metaprogramming. Metaprogramming is a technique that allows us to write code that returns code that we can use. We will start with a simple example.

class Simple
  def hello
    puts "Hello"
  end
  
  def method_missing(name, *args, &block)
    puts "tried to handle unknown method %s" % name
    unless args.empty?
      puts "it had arguments: %p" % [args]
    end
  end  
end

We can try loading that class in irb and running a few methods on it.

irb(main):013:0> Simple.new.hello
Hello
=> nil
irb(main):014:0> Simple.new.goodbye
tried to handle unknown method goodbye
=> nil
irb(main):015:0> Simple.new.goodmorning("Julio")
tried to handle unknown method goodmorning
it had arguments: ["Julio"]
=> nil

The method_missing method catches any method that we attempt to call in an instance of the Simple that is not previously declared. If the method does not exist in Simple then method_missing will be called with all of the details of the non-existent method and print its values to the stdout.

This provides us a way to handle methods we have not defined within in the class. We can use particular details about the method name, arguments or code block to decide what to do next.

Background

Before exploring Metaprogramming further, we need to understand the Ruby object system and introspection. In Ruby, we do everything in the context of an object. Even when we run irb and run code at the top level, we are actually working in main of class Object. At anytime you add methods to a ruby class or override existing methods.

class and instance variables

Class variables are available directly from a class to a class or an instance. Instance variables require making an instance with new and then the instance have access to the variables. Users only have direct access to exposed class and instance variables. However, you can still access private variables through Object methods. Class variables only have one value per class and subclass hierarchy.

class Animal
  @@legs = 2
  def self.legs
    @@legs # class variable getter
  end
end

puts Animal.legs # => 4

Even when you inherit a class with class variables and change the value in the subclass, the value update will be reflected in the parent class as well. The class variable is shared between the parent and the subclasses.

class Dog << Animal
  @@legs = 4
end

puts Animal.legs # => 4
puts Dog.legs # => 4

We will remake the dog class with its own class and instance variables. We expose the @name instance variable via attr_accessor.

class Dog
  @@legs = 4
  @name 
  attr_accessor :name # create getter and setter for Dog  
end

d = Dog.new
d.name = "Fido"
puts d.instance_variables # => [:@name]

class and instance methods

Class methods are available without making an instance and require the self prefix. They cannot access instance variables. Instance variables are available after creating an instance with new. They can access class and instance variables.

class Foo
  def self.bar
    puts 'class method'
  end
  
  def baz
    puts 'instance method'
  end
end

include

The include statement mixes in a module into a class. It is a simple form of multiple inheritance and forms a “is-a” relationship. It makes its methods available to an instance of the class that includes it.

module Foo 
  def foo 
    puts "Hello from Foo"
  end
end

class Bar
  include Foo
end

Bar.new.foo
Hello from Foo

extend

The extend statement makes a module’s methods to class. We can call the methods directly from the class without making a new instance.

class Baz
  extend Foo
end

Baz.foo
Hello from Foo

self

self is a contextual pointer. It points to the current object the program is located in. When we are in the top level of the file, we are located in the main object.

> puts self
main

> puts self.class
Object

When we declare a method at the top level, it becomes part of the main object. We can see that :m is a method of main.

def m
end

puts self.method(:m)
=> #<Method: Object#m>

self.methods
=> [:m ...]

In an object or module self is the current object.

class S
  def put_self
    puts self
  end
end

> S.new.m
#<S:0x007fad47294db0>

module Library
  puts self
end
Library

BasicObject

BasicObject¹ is the parent object of all objects in Ruby. It can be used to create alternative object hierarchies in Ruby. The main hierarchy in Ruby is Object that includes Kernel. It provides a small number of methods.

There are four callback methods that are useful for metaprogramming.

method_missing
singleton_method_added
singleton_method_removed
singleton_method_undefined

Kernel

Kernel² provides a number of useful methods such as Array, Hash, Integer, String, open, puts, etc. Kernel is mixed into Object and main is of class type Object so all of these methods act as global methods in Ruby.

These methods are useful for metaprogramming.

eval
exec
lambda
local_variables

Object

Object³ is the main object in the Ruby object hierarchy. It includes Kernel and is a child of BasicObject. Object has a large number of useful methods for metaprogramming.

define_singleton_method     extend                    instance_of? 
instance_variable_defined?  instance_variable_get     instance_variable_set
instance_variables          is_a?                     kind_of?
method                      methods                   private_methods
protected_methods           public_method             public_methods
public_send                 remove_instance_variable  respond_to?
respond_to_missing?         send