A playground is not truly a program. While it does execute code like a program, it is not really useful outside of the development environment. Before we can understand what the playground is doing for us, we must first understand how Swift works.

Swift is a compiled language, which means that for Swift code to be run, it must first be converted into a form that the computer can actually execute. The tool that does this conversion is called a compiler. A compiler is actually a program and it is also a way to define a programming language.

The Swift compiler takes the Swift code as input and, if it can properly parse and understand the code, outputs machine code. Apple developed the Swift compiler to understand the code according to a series of rules. Those rules are what define the Swift programming language and those rules are what we are trying to learn, when we say we are learning Swift.

Once the machine code is generated, Xcode can wrap the machine code up inside an app that users can run. However, we are running Swift code inside our playground, so clearly building an app is not the only way to run code; something else is going on here.

Every time you make a change to a playground, it automatically tries to compile your code. If it is successful, instead of wrapping up the machine code in an app to be run later, it runs the code immediately and shows you the results. If you had to do this process yourself, you would first have to consciously make the decision to build the code into an app and then run it when you wanted to test something. This would be a huge waste of time; especially, if you write an error that you don't catch until the moment you decide to actually run it. The quicker you can see the result of a code change, the faster you will be at developing the code and the fewer mistakes you will make.

For now, we will be developing all of our code inside a playground because it is a fantastic learning environment. Playgrounds are even more powerful than what we have seen so far and we will see that as we explore deeper into the Swift language.

We are just about ready to get to the meat of learning Swift, but first let's take a moment to make sure that you can get the most out of this book.