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The Power of Interfaces in Ruby

Igor Sarcevic Igor wrote this in January 2017

Ruby is not the fastest language, nor is it the simplest language, but damn it, it is definitely one of the most fun languages out there. For someone like me, who enjoys programming maybe a bit too much, a language that puts little or no limitations on the things I can tweak feels very natural and an obvious choice when it comes to solving complex issues.

While I really do love the Ruby language, sometimes I find myself missing features that are available elsewhere. For example, lightweight threads like in Go, or easy access to memory like in C.

Lately, I started missing interfaces from Java.

Interfaces

I was never a big fan of Java, the syntax is a bit too verbose for my taste. However, I always found the concept of interfaces interesting. I think that they are an excellent tool to communicate intent and to set up constraints on the objects that implement them.

For example, in Java, if I want to say that an object can be transformed to and from CSV (comma separated values) I could simply declare a CSV interface.

interface CSV {
  public void fromCSV(String line);
  public String toCSV();
}

Now, I can be sure that every object that implements that interface can be transformed to and from a CSV file. Here are two examples:

class User implements CSV {
  public String toCSV() {
    return this.name + "," + this.age + "," + this.password;
  }

  public void fromCSV(String csvLine) {
    String[] parts = csvLine.split(",");

    this.name = parts[0];
    this.age = parts[1];
    this.password = parts[2];
  }
}

class Book implements CSV {
  public String toCSV() {
    return this.title + "," + this.author;
  }

  public void fromCSV(String csvLine) {
    String[] parts = csvLine.split(",");

    this.title = parts[0];
    this.author = parts[1];
  }
}

Some people argue that interfaces are not important in Ruby, and that I should simply embrace the language and rely on duck typing.

This is however in direct opposition of what Ruby is all about. The Ruby community is well known for not accepting the status quo, and not limiting itself by what is currently available in the core of the language.

Interfaces in Ruby with modules

The simplest way to emulate interfaces in Ruby is to declare a module with methods that raise a “not implemented” exception.

module CSV

  def to_csv
    raise "Not implemented"
  end

  def from_csv(line)
    raise "Not implemented"
  end

end

Now, we can use that module to communicate what needs to be implemented on the objects in order to support CSV transformations.

class User
  include CSV

  def to_csv
    "#{@name},#{@age}"
  end

  def from_csv(line)
    parts = line.split(",")

    @name = parts[0]
    @age = parts[1]
  end

end

This looks and feels like interfaces in Java, however there is a huge downside. If we include the CSV module and forget to implement the to_csv method, we won’t notice this mistake until we run our code and an exception pops up.

This completely defeats the purpose of interfaces. I want to catch inconsistencies before I execute my code. If the CSV module can’t guarantee that, than what is its purpose? Does it act like a simple todo list for the methods I need to implement, without any actual checks?

Describing behaviour

Instead of focusing on the syntax, it is probably better to focus on the semantics of interfaces. What I actually want, is a set of constraints that define the behaviour of an object, and check them before I run my application.

When we put it that way, it is very obvious that what I want is actually a set of tests that describe my object. For the previous CSV example, we could write the following in RSpec:

shared_examples "a CSV serializable object" do
  it { is_expected.to respond_to(:to_csv) }
  it { is_expected.to respond_to(:from_csv) }
end

This is exactly the constraint that I expect from my object.

We can use the above definition as a substitute for interfaces in Ruby. If we want to enforce that an object can be transformed to and from CSV, we can simply drop a line in their specs:

describe User do
  it_behaves_like "a CSV serializable object"
end

describe Book do
  it_behaves_like "a CSV serializable object"
end

Tests are better than interfaces

Many things that are traditionally enforced with type systems are checked via unit tests in dynamically typed languages.

Type systems can be helpful, but it is very hard to define a type system that is simple enough, and at the same time strong enough to actually help the developer to write correct code. From my experience, type systems are an excellent tool to let the compiler know where to optimize your code, but I still am not convinced that it actually helps the developers to write better code.

The same is true for interfaces. They are able to describe the behaviour of an object, but they are fundamentally limited. Unit tests can describe much, much more about an object.

For example, to describe the behaviour of objects that act as collections, we can write down the following:

shared_examples "a collection" do
  it { is_expected.to respond_to(:add).with(1).argument }
  it { is_expected.to respond_to(:remove).with(1).argument }
  it { is_expected.to respond_to(:include?).with(1).argument }
end

This is roughly equivalent to an interface in Java, but why stop here? We can write down a lot more things that we expect from a collection.

shared_examples "a collection" do
  it { is_expected.to respond_to(:add).with(1).argument }
  it { is_expected.to respond_to(:remove).with(1).argument }
  it { is_expected.to respond_to(:include?).with(1).argument }

  before do
    @collection = described_class.new
  end

  describe ".add" do
    it "adds an element into the collection" do
      @collection.add(12)

      expect(@collection).to include(12)
    end

    it "returns the collection" do
      expect(@collection.add(12)).to eq(@collection)
    end
  end

  describe ".remove" do
    context "the element is not present" do
      it "doesn't raise an exception" do
        expect { @collection.remove(12) }.to_not raise_exception
      end
    end

    it "removes all the elements from the collection" do
      @collection.add(12)
      @collection.add(12)

      expect(@collection).to include(12)

      @collection.remove(12)
      expect(@collection).to_not include(12)
    end

    it "returns the collection" do
      expect(@collection.remove(12)).to eq(@collection)
    end
  end
end

Just look at the above example, and realize how much stronger are the above tests compared to a simple interface. We have successfully enforced and communicated to fellow developers the following:

  • A collection must answer to add, remove and include? methods
  • The arity of the methods and their return values
  • The remove method removes all the values from the collection
  • No exception is raised when remove is called on an empty collection

Avoiding mistakes and trusting unit tests

Until now, I frequently used the “not implemented” exception to communicate intent to other developers in my team. I used it to define interfaces, and also to define virtual methods on base abstract classes.

# to define an interface

module JsonSerialization
  def to_json
    raise "Not Implemented"
  end
end

# to define virtual methods

class Vehicle
  def speed
    raise "Not Implemented"
  end
end

I now realize that this was a mistake. Instead of doing this, I should have relied on the implicit NoMethodError that is raised by default in Ruby, and to use tests to describe the behaviour of my objects.

That’s all.
Keep learning, Ruby is great.

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