Understanding playgrounds

A playground is not actually 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 itself a program and it is one way to define a programming language.

The Swift compiler accepts Swift code as input, and if it can properly parse and understand the code, it 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 what we are trying to learn when we say we are learning Swift.

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

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 have to perform 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 want 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 develop 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 this as we delve 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 ensure that you can get the most out of this book.