Optionals are values which can represent the absence of a value. Optionals have two cases: either there is a value, or there is nothing.
An optional type is declared using the ? suffix for another type. For example, Int is a non-optional integer, and Int? is an optional integer, i.e. either nothing, or an integer.
The value representing nothing is nil.
// Declare a constant which has an optional integer type,
// with nil as its initial value.
//
let a: Int? = nil
// Declare a constant which has an optional integer type,
// with 42 as its initial value.
//
let b: Int? = 42
// Invalid: `b` has type `Int?`, which does not support arithmetic.
b + 23
// Invalid: Declare a constant with a non-optional integer type `Int`,
// but the initial value is `nil`, which in this context has type `Int?`.
//
let x: Int = nil
Optionals can be created for any value, not just for literals.
// Declare a constant which has a non-optional integer type,
// with 1 as its initial value.
//
let x = 1
// Declare a constant which has an optional integer type.
// An optional with the value of `x` is created.
//
let y: Int? = x
// Declare a variable which has an optional any type, i.e. the variable
// may be `nil`, or any other value.
// An optional with the value of `x` is created.
//
var z: AnyStruct? = x
A non-optional type is a subtype of its optional type.
var a: Int? = nil
let b = 2
a = b
// `a` is `2`
Optional types may be contained in other types, for example arrays or even optionals.
// Declare a constant which has an array type of optional integers.
let xs: [Int?] = [1, nil, 2, nil]
// Declare a constant which has a double optional type.
//
let doubleOptional: Int?? = nil
