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16 June 2013
The Rise of the Gang of Four with Rust
Since I’ve been to Seattle, I’ve had a lot of time to catch up on reading. One of the books that I’ve been reading has been well regarded since it was released in 1994.
Design Patterns: Elements of Reusable Object-Oriented Software (also called the Gang of Four book) is considered to be one of the best books when it comes to design patterns. It is often touted as a classic and I always approach such books skeptically. However, the first two chapters absolutely blew me away. This post is mainly about the first chapter and how it has changed my view of programming languages as I took up learning Rust.
The Standard Object-Oriented Paradigm
Before I read this book, when I thought of object-oriented programming (OOP), the languages that I thought of were usually Java, C++, Python, and Ruby (and according to GitHub, they are the four most popular OOP languages, besides PHP) The interesting part is that all four of these languages use the same notion of OOP with little twists added on.
Using these four languages, let’s look at some common terms that are used when trying to understand OOP for the first time.
Also, feel free to skim through this section on the standard paradigm if you are familiar with OOP in Java, C++, Python, and Ruby. The crux of this post is in the next section on the new paradigm.
Types & Classes
A type is a name given to an object to distinguish what kind it is. In OOP
languages, there are many different kinds of types. With Java, the types can be
primitive types, which means they are built into the language, like
boolean. Or they can be names given to the class an object is. Here’s what
determining an object’s type looks like in Python:
Now in Java:
As you can see, the idea of a type is used in these languages as a one-to-one mapping between types and objects. In other words, an object can only have one type.
I group the terms type and class together because they can be pretty interchangeable when talking about objects. However, sometimes a class is only referred to when talking about non-primitive objects.
Using our example, a class would only refer to
primitives such as
boolean have no class.
A signature is the term used to describe what a function or method is called, what it takes in as parameters, and what it returns.
In Ruby signatures look like this:
Ruby is dynamically typed so none of the variable types need to be mentioned. Java on the other hand is not dynamically typed. Thus a signature in Java looks like this:
The signature in Java looks different than the one in Ruby because the types
need to be declared. The first type we see is that the method called
int and takes two
ints as arguments.
An interface is a set of signatures that an object must respond to when implementing it. This is a way of guaranteeing that an object has a method defined for it.
The only programming language with explicit support for interfaces is Java. One can create an interface with C++ by just defining a class without providing any code. A similar thing can be achieved in Python by creating a class but just throwing a NotImplementedError as well as Ruby as explained here (although using “interfaces” in Python and Ruby could arguably be a bad thing).
Java interfaces look like this:
Now any object that implements the
Sloth interface is guaranteed to respond to
the two methods:
Polymorphism is considered to be one of the tenets of OOP. Polymorphism is when an object can take on multiple forms. This sounds strange but you’ve already seen an example of this but let’s look at it more explicitly.
As you can see, when we create a
DragonTrainer, the trainer doesn’t have to know
FireBreatingDragon, it only has to know about the class
happens is that since
Dragon, it just gets casted
to it during the
train method and calls the appropriate method.
Inheritance is almost exactly what it sounds like, if one has an object that extends from another, it will now have all of the same signatures and instance variables.
Taking our Dragon example from earlier, if we left off the implementation for
roar method in our
FireBreatingDragon, then it would just output “Roar!”
again because it inherits the
roar method from
An Object-Oriented Paradigm Shift
The Gang of Four book starts off in Chapter One by defining each of the terms I have listed above. As I read it, I figured it would all be review, yet I was getting very confused by the definitions. I re-read the chapter and my mind was forever changed.
Let’s take a look at each of those definitions from earlier and see how they compare. I’ve switched around the order a bit to aid in explanation.
In the Gang of Four, a signature is exactly what it is in Java, Python, and others. A signature still refers to the name of a method/function, its return type, and its parameters. Look above for an example.
Once again, an interface has no difference either. It is still a set of signatures that an object must respond to.
You may be wondering now why I thought that the Gang of Four was such a mind-blowing read, well hopefully the next set of terms as well as the examples in Rust will explain that.
Types & Classes
In the Gang of Four, a type is just a name to denote a particular interface. If an object has a certain type, then it responds to all the requests for that interface.
This doesn’t seem that revolutionary, but what does this imply? Well this means that an object can have multiple types. An object can actually have as many types as it wants, it just needs to implement more interfaces.
Let’s take a look at what this looks like in Rust. If your Rust is a little rusty (heh heh) or if you don’t know it, take a look at the tutorial that is provided. It is an excellent introduction to the language. Anywho, here’s what it looks like:
The first thing that you probably noticed is the keyword
trait, this is what
Rust calls a set of signatures, also known as an interface. Then we implement
those interfaces in the
impl section based on the
This paradigm is completely different than the standard idea of a type that is used in Java, C++, Python, and more.
Next is the idea of a class. In the Gang of Four, a class is just an object’s implementation. A class also specifies the object’s internal data, representation and it defines the operations the object can perform.
Although Rust doesn’t use the keyword
class, following the definition in the
last paragraph, a class could best be described as a
implementations. Although it isn’t a class in the traditional sense (there is no
class based inheritance), it can sort of be viewed as one (better analysis on
classlessness in Rust as well as Go)).
In Rust the idea of polymorphism is made possible all through the type system. The forms that an object can take are all dependent on the types of an object, in other words, the interfaces it implements.
As I mentioned, Rust doesn’t have the idea of class based inheritance. Instead it has the idea of trait inheritance.
When creating a trait, it’s possible to inherit other traits to extend functionality.
This example shows that if we create a
Shape trait that has one signature, an
area method, we can then use that trait and implement it for subsequent
structs. Since the area formula for a
Rectangle is different than a
we can represent that easily with type inheritance by creating a new type called
Round. Since the
Round trait inherits the
Shape trait as well, we need to
implement both traits for the
Rust is only at version 0.6. Mozilla wants to get a stable 1.0 version out by the end of the year so there is no telling what might change.
The interesting thing is that Rust is following the same pattern that Go took as well. Go is very similar to Rust in that it has interface inheritance and no classes.
Although the Gang of Four book certainly wasn’t the first to come up with the definitions of an interface, type, and class, it was the first place I came across the new ideas. Since these ideas are a radical shift from the previous paradigm, I can’t help but think that the Gang of Four has influenced Rust and Go, the two languages that I’m most excited to see evolve. It’s often said that “history repeats itself”, yet I can’t help but feel that great design also follows the same cyclic pattern.