Even if you’re not new to ruby, the concept of ducktyping might evokes something vague and unclear to you. I personally had to re-read the dedicated section of my ruby book a couple of times before I finally grasped the concept.

What is ducktyping?

Ducktyping is the art of judging an object on what it can do rather than on what it is believed to be. What it means is that an object doesn’t live and die with a label pinned on it’s forehead. In ruby, an object can do anything it is able to do, isn’t it great? Yeah, it is.

It makes a lot of sense when you think about it. Suppose you have a method called be_scary that accepts one parameter representing a lion that will roar at the call of the roar! method.

def be_scary(the_lion)
  the_lion.roar!
end

Even if the creator of this method seemed to have forget it, there are a lot of other living creatures on the earth that can express their might like that. No need to remind the programmer about that however, because we can use ducktyping to pass any scary stuff to our method. The thing that I will pass to the be_scary method do not need to share anything in common with a lion excepts for the fact that it can roar. Imagine for a second that scientists just discovered that rabbits could roar in some very specific conditions. No worries! You don’t have to touch a single line of your ruby program structure. Just pass the rabbit instance to the be_scary method and the interpreter will try to find it’s roar! method. Ruby doesn’t play a bouncer role like in many other languages. It doesn’t stand at the method gates while refusing the entrance to objects who are not exactly what the method is expecting to see. Instead, the ruby interpreter slacks off and let everyone enters into the method so they can try their luck with the roar! method.

Isn’t it the same thing than dynamic typing?

No, dynamic typing just makes reference to the way the type(or class) of an object is determined, that is, when you’ll start putting stuff in it. Ducktyping picks up from there and extends the capabilities. Generally, when ducktyping occurs, the interpreter knows the type of an object but still won’t exert authority on it by prohibiting it to enter some methods. We could say that a ducktyped language is always a dynamically typed language but not the other way around. A dynamically typed but not ducktyped language could still refuse objects based on their type at the doors of a method.

If you really want to catch exceptions before the ruby interpreter…

If you don’t feel comfortable with the fact that there is no bouncer at the doors of your method, you’ll be happy to know the existence of the respond_to? method. It kinds of fill the role of the typeof keyword in some other languages. However, instead of verifying if the object is a specific thing, you verify that it can do a specific thing.
See how we could have implemented our be_scary method.

def be_scary(something)
  if(something.respond_to?(:roar!))
    something.roar!
  else
    raise "That thing cannot roar..."
  end
end

Great… but if the only thing you are going to do is raise an exception (something ruby would have done by itself), this kind of practice may be a little overkill. It’s up to you to decide if the extra effort is worth it or not.

Ducktyping is not polymorphism

The beauty of ducktyping is that it doesn’t rely on inheritance or polymorphism at all. Every single objects are accepted in every single methods, period. The only thing that matters is the capability of an object to do whatever the method is trying to do. If the object can’t, the interpreter will gladly raise an exception in time (unless you use some respond_to technique that is).

Give ducktyping a real chance

Ducktyping can shake the conceptions you have about types and classes in programming languages. At first, I was not sure if I liked the idea, but now I think that it is a very logical and clever way to manage types. I came to the realization that programming languages didn’t need that many boundaries after all. I even wonder why no one thought about ducktyping before. Where’s the catch?