What are optionals used for in the languages that support them?

Swift is a new programming language for iOS, OS X, and watchOS app development. Nonetheless, many parts of Swift will be familiar from your experience of developing in C and Objective-C.

Swift provides its own versions of all fundamental C and Objective-C types, including Int for integers, Double and Float for floating-point values, Bool for Boolean values, and String for textual data. Swift also provides powerful versions of the three primary collection types, Array,Set, and Dictionary, as described in Collection Types.

Like C, Swift uses variables to store and refer to values by an identifying name. Swift also makes extensive use of variables whose values cannot be changed. These are known as constants, and are much more powerful than constants in C. Constants are used throughout Swift to make code safer and clearer in intent when you work with values that do not need to change.

In addition to familiar types, Swift introduces advanced types not found in Objective-C, such as tuples. Tuples enable you to create and pass around groupings of values. You can use a tuple to return multiple values from a function as a single compound value.

Swift also introduces optional types, which handle the absence of a value. Optionals say either “there is a value, and it equals x” or “there isn’t a value at all”. Optionals are similar to usingnil with pointers in Objective-C, but they work for any type, not just classes. Optionals are clear about the types of values your code can work with. If part of your code expects aString, type safety prevents you from passing it an Int by mistake. This restriction enables you to catch and fix errors as early as possible in the development process.

Functions

After the rules of syntax and semantics, the three most basic components of all Lisp programs are functions, variables and macros. You used all three while building the database in Chapter3, but I glossed over a lot of the details of how they work and how to best use them. I'll devote the next few chapters to these three topics, starting with functions, which--like their counterparts in other languages--provide the basic mechanism for abstracting, well, functionality.

The bulk of Lisp itself consists of functions. More than three quarters of the names defined in the language standard name functions. All the built-in data types are defined purely in terms of what functions operate on them. Even Lisp's powerful object system is built upon a conceptual extension to functions, generic functions, which I'll cover in Chapter.

And, despite the importance of macros to The Lisp Way, in the end all real functionality is provided by functions. Macros run at compile time, so the code they generate--the code that will actually make up the program after all the macros are expanded--will consist entirely of calls to functions and special operators. Not to mention, macros themselves are also functions, albeit functions that are used to generate code rather than to perform the actions of the program.1

Defining New Functions Function Parameter Lists Optional Parameters Rest Parameters Keyword Parameters Mixing Different Parameter Types Function Return Values Functions As Data, a.k.a. Higher-Order Functions Anonymous Functions