W1035 Not a Value

Within these castle walls be forged Mavens of Computer Science ...

— Merlin, The Coder

Paris, Notre Dame, Kilometer Null

Prerequisites

• W1031 Positive Integers
• W1032 Negative Integers
• W1033 Character Encoding

Introduction

There's often a need to represent the absence of a value. For example, consider a database tracking students' first name, middle name, and last name. Not all students have a middle name. So what value should we store in a variable representing a student's middle name if this particular student doesn't have one?

The name of this special value, representing non-existence, varies from language to language. In Swift, it's called nil.

Nil

Nil represents the absence of value of the specified type. We indicate that we intend to accept such nil values by specifying the type explicitly and suffixing the type name with a question mark. For example, in the aforementioned case, rather than specify the type as:

let middleName : String

we use:

let middleName : String?

This enables us to specify either a character or the special place-holder for no value, nil:

let middleName : String? = "A"

-or-

let middleName : String? = nil

Types which may be nil are termed optionals, because the constant or variable might have a value of the specified type; if not, it will be nil.

Nil Complexities

While nil is very helpful in these cases, using the value in an expression becomes slightly more complex, because for each such value, we need to take into account the possibility that it may be nil. Let's consider another example:

let a : Double = 0.4
let b : Double? = nil
let c = a + b

Line 3 will result in an error: value of optional type 'Double?' must be unwrapped to a value of type 'Double'