Keywords with and without @ in Swift

Question

In Swift, sometimes, keywords are plain keywords, and some others start with an @.

For instance, weak, unowned, inout, class are plain. But @final, @lazy start with @.

Sometimes, we even have both! prefix and @prefix, infix and @infix for instance.

It is not entirely an Objective-C inheritance since we have @class and not class in Objective-C. I could understand why we have class and not @class in Swift, but since we have @final or @lazy , I would have thought that it should be @weak and not weak.

Why this choice? Is there a kind of intuitive way that should tell: "hey, it is logical that this keyword starts with @?

Even if I think with a preprocessor perspective in mind, it is not obvious that @ would call a kind of specific preprocessor before compilation (e.g. @final is not really a kind of preprocessor directive).

Answer 1

@-prefixed items in Swift are not keywords, these are attributes.

Apple's book on Swift says that

Attributes provide more information about a declaration or type. There are two kinds of attributes in Swift, those that apply to declarations and those that apply to types.

Some attributes (such as @objc(isEnabled)) accept parameters.

The main difference between attributes and keywords is that keywords tell the compiler what you are defining (a class, a method, a property, a variable, and so on), while attributes tell the compiler in what contexts you intend to use that definition. For example, you would use a func keyword to tell the compiler that you are defining a function, and decorate that function with an @infix attribute to tell the compiler that you plan to use that function as an infix operator.