Wait…. Whaaaaaaaaaaaaaaaaaaat?
I don’t know about you, but that was my exact reaction when I first heard Nadia Odunayo
’s talk in RubyConf KL 2018
(mind you, one of the most captivating talk I’ve ever heard!), in which she showed that that Ruby has no class methods.
When I first started learning Ruby, I’ve been told that Ruby basically have two types of methods:
- Class Methods
- Instance Methods
class Foo
def self.class_method
puts "This is a class method of #{self}"
end
def an_instance_method
puts "This is an instance method of #{self}"
end
end
Foo.class_method #=> "This is a class method of Foo"
Foo.new.class_method #=> undefined method `class_method' for #<Foo:0x007f83960d2228> (NoMethodError)
Foo.an_instance_method #=> undefined method `an_instance_method' for Foo:Class (NoMethodError)
Foo.new.an_instance_method #=> This is an instance method of #<Foo:0x007f80aa92d150>
Well at first, it makes a lot of sense —
A class is an object, and when you call
.new
on a class, you create a new instance of that object.Hence, instance methods are only available on instances of that class.
Okay so then.. what am I talking about when I said Ruby has no class methods?
Parents. Grandparents. Ancestors..!
First thing we need to understand is something called Ancestor Chains in Ruby.
Ancestors Chains is vital to understanding how Ruby works; the entire method calling chain works because of the chain, but here I’ll attempt at an overly simplified explanation.
In Ruby, everything inherits Object
, and every Object
inherits from a BasicObject
.
Let’s do a quick check!
2.3.1 :001 > Foo.ancestors
=> [Foo, Object, Kernel, BasicObject]
2.3.1 :002 > Foo.superclass #superclass allow us to see what class the receiver inherited from. AKA, the parent of current class.
=> Object
2.3.1 :003 > Object.superclass
=> BasicObject
(Kernel is a Module, you could quickly verify it by doing Kernel.class)
When you call a method, it calls the method on the current class; if the current class does not have that method, it goes up the ancestor chain until it finds one.
Not clear yet? I’ve gotchu! Cue weird metaphor
Imagine you’re just a kid, and that annoying Little Johnny who is trying to outsmart you asks you sheepishly: “Do you know what
.planets
are in our solar system? I bet you don’t!”.Well you actually don’t know the answer, but you wouldn’t let Little Johnny get his victory, and so you make this your life goal to find out, and you ask your parents: “What are the
.planets
in our solar system?”. And well, your parents don’t know that either, so they go on and ask their parents, who happen to know the answer.Now it’s your turn to smirk sheepishly back at Little Johnny and tell him what the answer is.
So now whenever someone asks you what are the
.planets
in our solar system, you have the answer, from your Grandparents.
How is this relevant? Well, we talked about how method lookup work, and in my last post I talked about reusing methods (Module
) with include
, extend
and prepend
. Now let’s look at their respective ancestor chains when you include
, extend
and prepend
a module.
Awesome = Module.new
class IncludeModule
include Awesome
end
IncludeModule.ancestors #=> [IncludeModule, Awesome, Object, Kernel, BasicObject]
class ExtendModule
extend Awesome
end
ExtendModule #=> [ExtendModule, Object, Kernel, BasicObject]
# WHERE IS THE MODULE?!
class PrependModule
prepend Awesome
end
PrependModule.ancestors #=> [Awesome, PrependModule, Object, Kernel, BasicObject]
You’ll realize something strange:
- When you
include
, the module is being inserted after the current class, which means it’ll be the first thing you hit when you can’t find a method on current class. -
prepend
is the opposite ofinclude
; it inserts before, so technically, you still wouldn’t be able to call it. - You don’t see the module on the ancestor chain when you extend a module
Hold on, we know for a fact that when we extend
a module, we get access to class methods, and we also know that method calling happen via ancestor chains, but.. how can extend
work if the module is not on the ancestor chain?
The reason is because of something in Ruby called Singleton Class (sometimes referred to as Metaclass
, Anonymous Class
or Eigenclass
)
Whenever you initialize a class in Ruby, two objects are generated — Foo
itself, and Foo’s Singleton Class
. To prove this, in her talk Nadia showed a great way to inspect the current count of classes using ObjectSpace
. ObjectSpace
is basically a way for you to interact with all currently living Ruby objects in memory.
ObjectSpace.count_objects[:T_CLASS] #=> 920
Foo = Class.new
ObjectSpace.count_objects[:T_CLASS] #=> 922
Now, before we understand what extend
is doing, we need to first understand Singleton Class
, so bear with me, get through the next section and we’ll then be able to understand the extend
magic!
The one about Singleton Class
Singleton Class
, like its alternative name, Anonymous Class
suggests, is anonymous, thus does not show up in the ancestor chain(!!!) That doesn’t mean we can’t access the singleton class though, in fact, there’s a handy method called .singleton_class
that we can use.
class Foo
end
Foo.singleton_class #=> #<Class:Foo>
Let’s take a look at what is the .class
of the class you just created.
class Foo
end
Foo.class #=> Class
As you may have noticed earlier, there's also an alternative way to initialize a class as below:
Foo = Class.new
Looks really familiar now doesn’t it? Foo is an instance of Class.
Why is this important? Because this means that the normal Class Methods that you have come to think you’re executing on Foo
is in fact, an instance method on Class
. — It works because Foo
is an instance of Class
!
We now know singleton classes aren’t visible on ancestor chains. Remember what else wasn’t visible on the chain? The module when you do an extend
!
We can check the ancestor chain of the singleton class itself.
HiddenModule = Module.new
class Foo
extend HiddenModule
end
Foo.singleton_class.ancestors #=> [#<Class:Foo>, HiddenModule, #<Class:Object>, #<Class:BasicObject>, Class, Module, Object, Kernel, BasicObject]
# Well HiddenModule is not so hidden anymore!
We can also prove that what we come to know as class_method
, is actually just an instance method on the singleton class.
module Mystery
def resolved?
true
end
end
class Foo
extend Mystery
end
Foo.resolved? #=> true
Foo.singleton_class.instance_methods.grep(/resolved?/) #=> [:resolved?]
Foo.method(:resolved?) #=> #<Method: Class(Mystery)#resolved?>
So now we know what extend
actually does behind the scene is basically an include
on the singleton class itself. Let’s prove it!
class Bar
Bar.singleton_class.include Mystery
end
Bar.resolved? #=> true
Bar.singleton_class.instance_methods.grep(/resolved?/) #=> [:resolved?]
Bar.method(:resolved?) #=> #<Method: Class(Mystery)#resolved?>
Still works!
But yeah, writing the full form requires a staggering three more words, that’s a lot of work, so we’ve got extend
to save us that keystroke instead!
Author’s Note
I’ll be pumping out a few more articles, meanwhile do check-out my last post on included
, extended
and prepended
hooks!
Phew, that’s it for my second blogpost. Looking forward to learning and sharing more!
Top comments (10)
So, you've effectively proven that.
Haha yeah, I admit the title is a bit clickbaity :p but it's more so that for my own personal understanding, class method wasn't what I thought it was.
Yeah in Nadia's talk she talked briefly about Smalltalk too! There's just so much more to learn, it's exciting!
Can explain what MOP is?
Meta Object Protocol, oh man you're in for a treat!!!! Check this book
Interesting, I was playing with writing an OO programming language in Common Lisp, looks like this might be a good architecture for the object system.
Good to learn, thanks. Where can we watch the video you said?
Hey! I attended the RubyConf live, and I don't think they recorded anything, so frankly I'm not quite sure if a video exists.
Maybe you could start by looking at Nadia's Twitter or something?
EDIT: Hey man I took some time to dig and found it! nadiaodunayo.com/speaking/the-case...
I found it, thank you. Nice video.
nadiaodunayo.com/speaking/the-case...
Great explanation Edison!
Thanks! Means a lot to someone who's pretty new to this blogging thing haha
haha I found this post very interesting. Great work!