Advanced Ruby

Return values

In Ruby everything is an expression - it executes and returns some value, the result of it’s execution. Let’s take a look at a simple example

5 + 2        # => 7
"hello"      # => "hello"
"a" if true  # => "a"
"a" if false # => nil

class A; end # => ???

What is the result of the last expression? It could be obvious - "we defined class A". But not really. The fact we have defined a class is only an effect of the expression but not the result, result is a value returned from the expression itself.

class A; end # => nil

The result is simply nil. It’s similar to the expression "a" if false above. The condition is evaluated as false and there is no else so there is nothing to return, so the result is nil. In this case a class was defined, but there was nothing to be return, as the body of the class was empty, so the result is nil. Let’s modify the example to return a value

class A; 1; end            # => 1
class A; 1; "hello"; end   # => "hello"
class A; self; end         # => A

Before it was said that the return value of a method is the result of it’s last expression. And it’s obvious that in this example it’s very similar, and it can be generalized for all structures, even though sometimes it may not be obvious on the first sight.

the return value is the the result of the last expression

Context

In Ruby everything is executed in some context. This context is know as current object and is always represented by self.

self.class # => Object

class B
  self
end
# => Class

class A
  def call
    self
  end
end

A.new.call # => #<A:some number>

Class

Open classes

Unlike most languages, Ruby classes are open for modifications. Developers can modify behavior of classes defined by frameworks or Ruby itself. This technique is called Monkey patching.

class Clazz
  def call
    "A"
  end
end

class Clazz
  def call
    "B"
  end
end

Clazz.new.call # => "B"

What is a class?

Let’s start with a definition

classes are instances of class Class

and as mentioned many times before, everything in Ruby is an object … even a class.

class A
  def self.call
    "A"
  end
end

B = Class.new

def B.call
  "called"
end

A.call      # => "called"
B.call      # => "called"

A.object_id # => [some number]
B.object_id # => [some number]

A.class     # => Class
B.class     # => Class

Class A was defined using the class keyword and then a class method was defined. Class B was created by creating new instance of the Class class and the object was assigned to constant B. As both of those classes are objects, it’s possible to check it’s class and the ID of the object.

Inheritance

In Ruby classes can inherit from each other, though Ruby has only single-class inheritance - it’s not possible to inherit from multiple classes, only from one.

class A
 def call
   "called"
 end
end

class B < A
end

C = Class.new(B)

B.new.call # => "called"
C.new.call # => "called"

Mixins

When some class needs to inherit from multiple classes, it’s not possible, but Ruby provides a workaround through mixins. It is possible to include many Modules into a class the methods defined in those modules will become part of the lookup path as if they were defined in the class.

module Methods
  def call
    "called"
  end
end

class A
  include Methods
end

A.new.call # => "called"

Class introspections

Ruby allows many introspections on classes and many other objects.

There is method name defined on a class that returns the name of the current class.

Array.name # => "Array"

[].class.name # => "Array"

It’s possible to list methods of an object

class A
  def call
  end
end

A.new.methods # => array of methods

Methods

As everything else in Ruby even methods are instances of class Method.

Extracting methods

Sometimes it is useful to pass around only a method instead of the whole object. Ruby allows extraction of a method for later usege.

class A
  def call(arg1)
    self
  end
end

meth = A.new.method(:call) # => #<Method: A#call>

In the example method call from class A was "extracted". The method is still bound to the instance of class A and the method will be evaluated in the context of the object (self will be the instance). The method can be executed by calling the call method with appropriate arguments.

meth.call("some string") # => #<A:some_number>

Checking method existence

Because Ruby is a very dynamic language, it’s not possible to know in advance what kind of arguments will be received. In most cases the developer should not care what class the argument is, but whether the argument responds to a method.

Do not care what the object is, only care whether it behaves as expected.

This technique is called Duck typing.

class A
  def call
  end
end

a = A.new

a.respond_to?(:call) # => true
a.respond_to?(:wtf)  # => false

Dynamic method calling

Let’s define a class with a method, create an instance and call the method.

class A
  def call
  end
end

A.new.call

The method is called, but the develeoper had to know the name of the method beforehand … in the time the code is written. What if the method name is not known and there has to be some method called. Do not be surprised, this is very common use-case in Ruby.

class A
  def call(arg1)
  end
end

a = A.new
a.call("some string")
a.send(:call, "some string")

Well, not so identical. When you use the send method on an object, you effectively bypass the access modifiers.

Ruby has three access levels public is default, protected and private.

class A

  def public_method
  end

  protected

  def protected_method
  end

  private

  def private_method
  end

end

a = A.new

a.public_method     # => nil
a.protected_method  # => NoMethodError: protected method `protected_method' called ...
a.private_method    # => NoMethodError: private method `private_method' called ...

a.send(:public_method)      # => nil
a.send(:protected_method)   # => nil
a.send(:private_method)     # => nil

Defining methods programmatically

The way to define methods using the def keyword shown before is not the only one. It’s also possible to define method in a more dynamic way. It makes sense. We can inspect methods of an object, we can extract methods of an object and also call methods of an object in a dynamic way. To dynamically define a method use the define_method method of a class, however

Class.define_method is private

To get around this obstacle, it’s possible to use the send method and bypass the access modifier.

class A
end

a = A.new

logic = Proc.new do
  "data"
end

A.send(:define_method, :some_method_name, logic)

a.some_method_name # => "data"

Missing methods

Every object can define special method_missing method that is called whenever there is a call to undefined method on that object.

class A
  def method_missing(name, *args, &block)
    puts "method #{name} called with args #{args.inspect}"
  end
end

A.new.something("a") # => method something called with args ["a"]

Objects

Objects complement classes in a way that

objects define state and classes define behavior

Behavior id defined as a class, then an object is created for that class to hold the state. Every object has to be of some class.

Creating new object

To create an object of a class there is the new method on respective class.

class Dog
end

dog = Dog.new

Defining methods

In the example above many methods were defined in simple or more fancy styles. But let’s get back to the core and try to define a method

class A
  def call
  end
end

here we use def keyword to define method call. Where will def define the method? The answer is simple and complex

def defines methods into the nearest class

So in the previous example the nearest class is A. That is obvious from next example where the current context is returned and inspected

var = class A; self; end

var.class  # => Class
var.name   # => "A"

OK, so the the current context is a Class and thus is’t obvious that the nearest class is this class. Now let’s try to define a class method

class A
  def self.call
    "string"
  end
end

Where will Ruby define the method now?? It is a bit more complicated. To understand this, we have to explain something else first.

Eigenclass

To understand how Ruby works, we have to understand what eigenclasses are. Let’s start with simple definition

every object in Ruby has it's own eigenclass => an instance of Class

Note	eigen means "it’s own" in German

Why is this important? Because, however the eigenclasses are basically invisible to developers, they take an important part in method lookups.

When Ruby is trying to look up a method, it follows a basic chain (will be described a bit later). Important is, that before the class the object is linked to, there is the one more class - object’s eigenclass. Every single object in Ruby has it’s own eigenclass and because Classes are object as well, eigenclasses has their own eigenclasses as well.

The closest class to an object is not it's class but it's eigenclass.

Back to the example we were talking about

class A
  def self.call
    "string"
  end
end

to see it more clearly we can rewrite this example identically as

class A
end

def A.call
  "string"
end

these two expressions are identical. To understand why it is important to understand this

class A
end

scope = class A
  self
end

A == scope # => true

but back to the original question … where is the method going to be defined? In the context of the instance of the class A. The important part is the instance of. What is the closest class to an instance (object)? As stated above it’s its eigenclass. Now you might have guessed that from implementation point of view

there are no class methods in Ruby

What would be called a class method is only an instance method defined on the eigenclass associated with object representing the class.

So eigenclass is some stealth object that we can not see? Not really. Ruby has ways to access eigenclasses

eigenclass = class << some_object
  self
end

eigenclass = some_object.singleton_class

now that we can access eigenclasses, let’s see how we could define "class methods" (instance methods in the eigenclass).

class A
  def self.call
    "called"
  end
end

class B
  class << self
    def call
      "called"
    end
  end
end

D = Class.new
class << D
  def call
    "called"
  end
end

all those examples are identical.

Method lookups

Now that you know where and how are methods defined, lets see how methods are looked up. Let’s see how the class hierarchy looks for class

SomeClass -> Class -> Module -> Object -> BasicObject

and for objects

object -> SomeClass -> Object -> BasicObject

though in real it is a bit more complex as seen in this picture

Eigenclasses are not visible as classes of objects.

o1 = Object.new

def o1.meth
  "string"
end

o1.meth  # => "string"
o1.class # => Object

o2 = Object.new

o2.meth  # => undefined method `meth`
o2.class # => Object

This example shows that having two instances of same objects. Both can behave differently. Because in the case of o1 the method is stored in the eigenclass, that is not accessible by o2.

Eigenclasses are used when a specific behavior of an object is expected.

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Advanced Ruby

Return values

Context

Class

Open classes

What is a class?

Inheritance

Mixins

Class introspections

Methods

Extracting methods

Checking method existence

Dynamic method calling

Defining methods programmatically

Missing methods

Objects

Creating new object

Defining methods

Eigenclass

Method lookups

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Advanced Ruby

Return values

Context

Class

Open classes

What is a class?

Inheritance

Mixins

Class introspections

Methods

Extracting methods

Checking method existence

Dynamic method calling

Defining methods programmatically

Missing methods

Objects

Creating new object

Defining methods

Eigenclass

Method lookups