[Ruby] Equal is not always equal

The reason for this blog article is a question that dealt with the different ways of checking equality in Ruby, or more specifically the so called “threequals” operator method. You might have come across it, it is the three equals sign ===, a very Ruby specific thing. Even though everybody calls it operator, I think for the understanding it is crucial to be specific here – “threequals” is a method (of an object), not an operator. I stress this, as this is not true for other object oriented languages, such as Java, in which all infix operators (such as ==) are simply part of the language, i.e. their definition exists outside the object world.

After the before mentioned question I struggled myself of finding any good explanation of the different methods Ruby provides of comparing things, which is why I decided to write down the things I told him; and additionally place it into the context of all four methods, Ruby offers. Yes, there are four ways of comparison, which is twice as much as languages such as Java or Smalltalk offer (and think of C++ which just knows just the == comparison). To fully appreciate the differences, let me start of with the first and typical stumbling block, every novice programmer encounters: value equality vs. reference equality (skip this if you are familiar with the concept).

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Java just won’t manage Non-Primitive Numbers

I just stumbled over something really funny; something that would fit to the Watman lightning talk by Gary Bernhardt. As you may know, Java is jet another language that copied a really great programming language of the visionary Alan Kay, named Smalltalk. In Smalltalk everything is an Object (yes, really really everything! So even classes are objects, describes by meta-classes – even creating a subclass is done by sending the superclass a message, to which it replies with a subclass!) and all programming is done by objects sending and receiving messages, and answering to them. This is why in OOP, you don’t have functions but methods. A function can be called – it can be applied to values. So you actually define what to do. In OOP, an object decides on how to properly react on a message. It does so by looking up it’s methods on how to react on the received message. But how it will react is totally up to the object, it’s even possible that many different objects reply to the same message by using different methods; a function on the other hand is unique. But the key idea is that while in imperative programming you see and apply the function, in object-oriented programming this is a black-box to you (unless you programmed the method).

But I digress. Coming back to Java, when it was developed, it wanted to implement OOP, but on the other hand it also wanted to keep up with the speed popularity of C. So not only did the syntax change to be more C-like, but also a lot of things that work purely object-oriented in Smalltalk where implemented as they are in C. This also applies to numbers, they are primitives; values that lie in RAM, values on which functions are applied to. Now, this may sound totally normal to us – we don’t think of a number as an object, which we ask to do something, and then see if it does it, or not. But with Smalltalk, that’s exactly how it’s done:

1 class
=> SmallInteger

1 resondsTo: #+
=> true

Other pure object-oriented languages, such as Objective-C or Ruby (example below) behave in a similar way:

=> Fixnum

=> true

So how does it work? + is a method, same as any other method – with the exception that it is written in a special way, so that it can be used more human-readable, in an infix notation. #+ and :+ are symbols in Smalltalk/Ruby, which are used to identify the method. Say you have a method called println(), the symbol would be :println in Ruby or #println in Smalltalk. Ruby allows us to send a message to a object not only by naming the method, but also by using the send()-Method, that every object understands, and where the first argument is the the symbol of the method and the following arguments are arguments accepted by the method. So, here it should become obvious that + is actually a method and the second number (another object) an argument:

1.send(:+, 2)
=> 3

The Answer to the message is actually a new object. Not so in Java. Primitives are non-object values


=> Unresolved compilation problem:
Cannot invoke getClass() on the primitive type int

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It’s somewhat of an oldtimer, though it’s only a year old – but time on web is fast, you know. A classic if you want. I guess 80 to 90 per cent of you guys have already seen it. But it’s a timeless classic, I’ve never blogged about it, and there are those 10 to 20 per cent that don’t know what I’m talking about, when I say Watman in a clearly computer science related environment.

So here we go. Enjoy this wonderful four minutes lightning talk by Gary Bernhardt on CodeMash 2012: