Contents

The Basics

Work with common kinds of data and write basic syntax.

Constants and Variables

Constants and variables associate a name
(such as `maximumNumberOfLoginAttempts` or `welcomeMessage`)
with a value of a particular type
(such as the number `10` or the string `"Hello"`).
The value of a *constant* can't be changed once it's set,
whereas a *variable* can be set to a different value in the future.

### Declaring Constants and Variables

Constants and variables must be declared before they're used.
You declare constants with the `let` keyword
and variables with the `var` keyword.
Here's an example of how constants and variables can be used
to track the number of login attempts a user has made:

```swift
let maximumNumberOfLoginAttempts = 10
var currentLoginAttempt = 0
```

<!--
  - test: `constantsAndVariables`

  ```swifttest
  -> let maximumNumberOfLoginAttempts = 10
  -> var currentLoginAttempt = 0
  ```
-->

This code can be read as:

“Declare a new constant called `maximumNumberOfLoginAttempts`,
and give it a value of `10`.
Then, declare a new variable called `currentLoginAttempt`,
and give it an initial value of `0`.”

In this example,
the maximum number of allowed login attempts is declared as a constant,
because the maximum value never changes.
The current login attempt counter is declared as a variable,
because this value must be incremented after each failed login attempt.

If a stored value in your code won't change,
always declare it as a constant with the `let` keyword.
Use variables only for storing values that change.

When you declare a constant or a variable,
you can give it a value as part of that declaration,
like the examples above.
Alternatively,
you can assign its initial value later in the program,
as long as it's guaranteed to have a value
before the first time you read from it.

```swift
var environment = "development"
let maximumNumberOfLoginAttempts: Int
// maximumNumberOfLoginAttempts has no value yet.

if environment == "development" {
    maximumNumberOfLoginAttempts = 100
} else {
    maximumNumberOfLoginAttempts = 10
}
// Now maximumNumberOfLoginAttempts has a value, and can be read.
```

<!--
  - test: `constantsWithDeferredInitialization`

  ```swifttest
  -> var environment = "development"
  -> let maximumNumberOfLoginAttempts: Int
  -> if environment == "development" {
         maximumNumberOfLoginAttempts = 100
     } else {
         maximumNumberOfLoginAttempts = 10
     }
  >> print(maxNumberOfLoginAttempts)
  << 100
  ```
-->

In this example,
the maximum number of login attempts is constant,
and its value depends on the environment.
In the development environment,
it has a value of 100;
in any other environment, its value is 10.
Both branches of the `if` statement
initialize `maximumNumberOfLoginAttempts` with some value,
guaranteeing that the constant always gets a value.
For information about how Swift checks your code
when you set an initial value this way,
see <doc:Declarations#Constant-Declaration>.

You can declare multiple constants or multiple variables on a single line,
separated by commas:

```swift
var x = 0.0, y = 0.0, z = 0.0
```

<!--
  - test: `multipleDeclarations`

  ```swifttest
  -> var x = 0.0, y = 0.0, z = 0.0
  >> print(x, y, z)
  << 0.0 0.0 0.0
  ```
-->

### Type Annotations

You can provide a *type annotation* when you declare a constant or variable,
to be clear about the kind of values the constant or variable can store.
Write a type annotation by placing a colon after the constant or variable name,
followed by a space, followed by the name of the type to use.

This example provides a type annotation for a variable called `welcomeMessage`,
to indicate that the variable can store `String` values:

```swift
var welcomeMessage: String
```

<!--
  - test: `typeAnnotations`

  ```swifttest
  -> var welcomeMessage: String
  ```
-->

The colon in the declaration means “…of type…,”
so the code above can be read as:

“Declare a variable called `welcomeMessage` that's of type `String`.”

The phrase “of type `String`” means “can store any `String` value.”
Think of it as meaning “the type of thing” (or “the kind of thing”) that can be stored.

The `welcomeMessage` variable can now be set to any string value without error:

```swift
welcomeMessage = "Hello"
```

<!--
  - test: `typeAnnotations`

  ```swifttest
  -> welcomeMessage = "Hello"
  >> print(welcomeMessage)
  << Hello
  ```
-->

You can define multiple related variables of the same type on a single line,
separated by commas, with a single type annotation after the final variable name:

```swift
var red, green, blue: Double
```

<!--
  - test: `typeAnnotations`

  ```swifttest
  -> var red, green, blue: Double
  ```
-->

> Note: It's rare that you need to write type annotations in practice.
> If you provide an initial value for a constant or variable at the point that it's defined,
> Swift can almost always infer the type to be used for that constant or variable,
> as described in <doc:TheBasics#Type-Safety-and-Type-Inference>.
> In the `welcomeMessage` example above, no initial value is provided,
> and so the type of the `welcomeMessage` variable is specified with a type annotation
> rather than being inferred from an initial value.

### Naming Constants and Variables

Constant and variable names can contain almost any character,
including Unicode characters:

```swift
let π = 3.14159
let 你好 = "你好世界"
let 🐶🐮 = "dogcow"
```

<!--
  - test: `constantsAndVariables`

  ```swifttest
  -> let π = 3.14159
  -> let 你好 = "你好世界"
  -> let 🐶🐮 = "dogcow"
  ```
-->

Constant and variable names can't contain
whitespace characters, mathematical symbols, arrows, private-use Unicode scalar values,
or line- and box-drawing characters.
Nor can they begin with a number,
although numbers may be included elsewhere within the name.

Once you've declared a constant or variable of a certain type,
you can't declare it again with the same name,
or change it to store values of a different type.
Nor can you change a constant into a variable
or a variable into a constant.

> Note: If you need to give a constant or variable the same name as a reserved Swift keyword,
> surround the keyword with backticks (`` ` ``) when using it as a name.
> However, avoid using keywords as names unless you have absolutely no choice.

You can change the value of an existing variable to another value of a compatible type.
In this example, the value of `friendlyWelcome` is changed from
`"Hello!"` to `"Bonjour!"`:

```swift
var friendlyWelcome = "Hello!"
friendlyWelcome = "Bonjour!"
// friendlyWelcome is now "Bonjour!"
```

<!--
  - test: `constantsAndVariables`

  ```swifttest
  -> var friendlyWelcome = "Hello!"
  -> friendlyWelcome = "Bonjour!"
  /> friendlyWelcome is now \"\(friendlyWelcome)\"
  </ friendlyWelcome is now "Bonjour!"
  ```
-->

Unlike a variable, the value of a constant can't be changed after it's set.
Attempting to do so is reported as an error when your code is compiled:

```swift
let languageName = "Swift"
languageName = "Swift++"
// This is a compile-time error: languageName cannot be changed.
```

<!--
  - test: `constantsAndVariables_err`

  ```swifttest
  -> let languageName = "Swift"
  -> languageName = "Swift++"
  // This is a compile-time error: languageName cannot be changed.
  !$ error: cannot assign to value: 'languageName' is a 'let' constant
  !! languageName = "Swift++"
  !! ^~~~~~~~~~~~
  !! /tmp/swifttest.swift:1:1: note: change 'let' to 'var' to make it mutable
  !! let languageName = "Swift"
  !! ^~~
  !! var
  ```
-->

### Printing Constants and Variables

You can print the current value of a constant or variable with the `print(_:separator:terminator:)` function:

```swift
print(friendlyWelcome)
// Prints "Bonjour!"
```

<!--
  - test: `constantsAndVariables`

  ```swifttest
  -> print(friendlyWelcome)
  <- Bonjour!
  ```
-->

The `print(_:separator:terminator:)` function
is a global function that prints one or more values
to an appropriate output.
In Xcode, for example,
the `print(_:separator:terminator:)` function prints its output in Xcode's “console” pane.
The `separator` and `terminator` parameter have default values,
so you can omit them when you call this function.
By default, the function terminates the line it prints by adding a line break.
To print a value without a line break after it,
pass an empty string as the terminator --- for example,
`print(someValue, terminator: "")`.
For information about parameters with default values,
see <doc:Functions#Default-Parameter-Values>.

<!--
  - test: `printingWithoutNewline`

  ```swifttest
  >> let someValue = 10
  -> print(someValue, terminator: "")
  -> print(someValue)
  << 1010
  ```
-->

<!--
  QUESTION: have I referred to Xcode's console correctly here?
  Should I mention other output streams, such as the REPL / playgrounds?
-->

<!--
  NOTE: this is a deliberately simplistic description of what you can do with print().
  It will be expanded later on.
-->

Swift uses *string interpolation* to include the name of a constant or variable
as a placeholder in a longer string,
and to prompt Swift to replace it with the current value of that constant or variable.
Wrap the name in parentheses and escape it with a backslash before the opening parenthesis:

```swift
print("The current value of friendlyWelcome is \(friendlyWelcome)")
// Prints "The current value of friendlyWelcome is Bonjour!"
```

<!--
  - test: `constantsAndVariables`

  ```swifttest
  -> print("The current value of friendlyWelcome is \(friendlyWelcome)")
  <- The current value of friendlyWelcome is Bonjour!
  ```
-->

> Note: All options you can use with string interpolation
> are described in <doc:StringsAndCharacters#String-Interpolation>.

Comments

Use comments to include nonexecutable text in your code, as a note or reminder to yourself. Comments are ignored by the Swift compiler when your code is compiled.

Comments in Swift are very similar to comments in C. Single-line comments begin with two forward-slashes (//):

// This is a comment.

Multiline comments start with a forward-slash followed by an asterisk (/) and end with an asterisk followed by a forward-slash (/):

/* This is also a comment
but is written over multiple lines. */

Unlike multiline comments in C, multiline comments in Swift can be nested inside other multiline comments. You write nested comments by starting a multiline comment block and then starting a second multiline comment within the first block. The second block is then closed, followed by the first block:

/* This is the start of the first multiline comment.
    /* This is the second, nested multiline comment. */
This is the end of the first multiline comment. */

Nested multiline comments enable you to comment out large blocks of code quickly and easily, even if the code already contains multiline comments.

Semicolons

Unlike many other languages, Swift doesn't require you to write a semicolon (;) after each statement in your code, although you can do so if you wish. However, semicolons are required if you want to write multiple separate statements on a single line:

let cat = "🐱"; print(cat)
// Prints "🐱".

Integers

Integers are whole numbers with no fractional component, such as 42 and -23. Integers are either signed (positive, zero, or negative) or unsigned (positive or zero), and their maximum and minimum value depends on their size (the number of bits used to store values). The integer types include their size and sign in their names --- for example, an 8-bit unsigned integer is of type UInt8, and a 32-bit signed integer is of type Int32. Like all types in Swift, these integer types have capitalized names. In most cases, when you don't need to specify the exact integer size, you use the Int type described below.

Integer types behave like most arithmetic you do by hand; integer math produces results without approximating. These characteristics make integers suitable for counting and other calculations that represent exact amounts --- for example, finding the longest line in a text file, applying a score multiplier in a game, or totaling prices on a receipt.

Although integers don't have a fractional component, you can use integers to represent quantities with fractions by counting a fractional part. For example, you can represent $1.23 by storing the number 123 in an integer that counts cents. This approach is known as fixed-point math because the decimal point is at a fixed position in the number. In the example above, the number 123 is understood to have a decimal point before the last two digits.

Note: For calculations in a regulated area like finance or construction, or in a domain that has an expectation of high-precision results, you might need a special-purpose numeric type that implements behaviors such as rounding and truncation, according to that area's requirements.

Integer Bounds

You can access the minimum and maximum values of each integer type with its min and max properties:

let minValue = UInt8.min  // minValue is equal to 0, and is of type UInt8
let maxValue = UInt8.max  // maxValue is equal to 255, and is of type UInt8

The values of these properties are of the appropriate-sized number type (such as UInt8 in the example above) and can therefore be used in expressions alongside other values of the same type.

Calculations that produce out-of-bounds results, like a number larger than the max property, stop the program's execution instead of storing an invalid result. You can explicitly make the operation overflow instead, as described in AdvancedOperators — Overflow Operators.

Int

In most cases, you don't need to pick a specific size of integer to use in your code. Swift provides an additional integer type, Int, which has the same size as the current platform's native word size:

  • On a 32-bit platform, Int is the same size as Int32.
  • On a 64-bit platform, Int is the same size as Int64.

Unless you need to work with a specific size of integer, always use Int for integer values in your code. This aids code consistency and interoperability. Even on 32-bit platforms, Int can store any value between -2,147,483,648 and 2,147,483,647, and is large enough for many integer ranges.

UInt

Swift also provides an unsigned integer type, UInt, which has the same size as the current platform's native word size:

  • On a 32-bit platform, UInt is the same size as UInt32.
  • On a 64-bit platform, UInt is the same size as UInt64.

Note: Use UInt only when you specifically need an unsigned integer type with the same size as the platform's native word size. If this isn't the case, Int is preferred, even when the values to be stored are known to be nonnegative. A consistent use of Int for integer values aids code interoperability, avoids the need to convert between different number types, and matches integer type inference, as described in TheBasics — Type Safety and Type Inference.

Floating-Point Numbers

Floating-point numbers have a fractional component, such as 3.14159, 0.1, and -273.15. Swift provides a variety of floating-point types that support different sizes of numbers, just like it has different sizes of integers. If you don't need to specify an exact size, use Double. Otherwise, use the type that includes the needed size in its name, such as Float16 or Float80. Following common terminology for floating-point math, Float uses 32 bits and Double uses 64 bits. You can also write these types as Float32 or Float64. For example, graphics code often uses Float to match the GPU's fastest data type. Some floating-point types are supported only by certain platforms, but Float and Double are available on all platforms.

Floating-point numbers let you work with very small and very large numbers, but can't represent every possible value in that range. Unlike integer calculations, which always produce an exact result, floating-point math rounds results to the nearest representable number. For example, when storing the number 10,000 as a Float, the next largest number you can represent is 10,000.001 ---- values between these two numbers round to one or the other. The space between numbers is also variable; there are larger spaces between large numbers than between small numbers. For example, the next Float value after 0.001 is 0.0010000002, which is smaller than the spacing after 10,000.

Floating-point numbers have values for negative zero, infinity, and negative infinity, which represent overflow and underflow in calculations. They also have not-a-number (NaN) values to represent an invalid or undefined result, such as dividing zero by zero. This behavior is different from integers, which stop the program if they can't represent the result.

If you need the same spacing between all possible values, or if the calculations you're doing require exact results and don't call for the special values listed above, a floating-point number might not be the right data type. Consider using fixed-point numbers instead, as described in TheBasics — Integers.

Type Safety and Type Inference

Every value in a Swift program has a type. Every place you store a value --- including constants, variables, and properties --- also has a type. You might write the type explicitly using a type annotation, or Swift might infer the type from an initial value. Every place in your code where you provide a value, that value's type must match the place you use it. For example, if part of your code requires a String, you can't pass it an Int by mistake. This kind of checking makes Swift a type-safe language.

A type safe language encourages you to be clear about the types of values your code works with. Values of one type are never implicitly converted to another type. However, some types can be explicitly converted. When building code, Swift checks the code for type safety and flags any mismatched types as errors.

Type checking helps you avoid errors when you're working with different types of values. However, this doesn't mean that you have to specify the type of every constant and variable that you declare. If you don't specify the type of value you need, Swift uses type inference to work out the appropriate type. Type inference enables a compiler to deduce the type of a particular expression automatically when it compiles your code, simply by examining the values you provide.

Because of type inference, Swift requires far fewer type declarations than languages such as C or Objective-C. Constants and variables are still explicitly typed, but much of the work of specifying their type is done for you.

Type inference is particularly useful when you declare a constant or variable with an initial value. This is often done by assigning a literal value (or literal) to the constant or variable at the point that you declare it. (A literal value is a value that appears directly in your source code, such as 42 and 3.14159 in the examples below.)

For example, if you assign a literal value of 42 to a new constant without saying what type it is, Swift infers that you want the constant to be an Int, because you have initialized it with a number that looks like an integer:

let meaningOfLife = 42
// meaningOfLife is inferred to be of type Int

Likewise, if you don't specify a type for a floating-point literal, Swift infers that you want to create a Double:

let pi = 3.14159
// pi is inferred to be of type Double

Swift always chooses Double (rather than Float) when inferring the type of floating-point numbers.

If you combine integer and floating-point literals in an expression, a type of Double will be inferred from the context:

let anotherPi = 3 + 0.14159
// anotherPi is also inferred to be of type Double

The literal value of 3 has no explicit type in and of itself, and so an appropriate output type of Double is inferred from the presence of a floating-point literal as part of the addition.

Numeric Literals

Integer literals can be written as:

  • A decimal number, with no prefix
  • A binary number, with a 0b prefix
  • An octal number, with a 0o prefix
  • A hexadecimal number, with a 0x prefix

All of these integer literals have a decimal value of 17:

let decimalInteger = 17
let binaryInteger = 0b10001       // 17 in binary notation
let octalInteger = 0o21           // 17 in octal notation
let hexadecimalInteger = 0x11     // 17 in hexadecimal notation

Floating-point literals can be decimal (with no prefix), or hexadecimal (with a 0x prefix). They must always have a number (or hexadecimal number) on both sides of the decimal point. Decimal floats can also have an optional exponent, indicated by an uppercase or lowercase e; hexadecimal floats must have an exponent, indicated by an uppercase or lowercase p.

For decimal numbers with an exponent of x, the base number is multiplied by 10ˣ:

  • 1.25e2 means 1.25 x 10², or 125.0.
  • 1.25e-2 means 1.25 x 10⁻², or 0.0125.

For hexadecimal numbers with an exponent of x, the base number is multiplied by 2ˣ:

  • 0xFp2 means 15 x 2², or 60.0.
  • 0xFp-2 means 15 x 2⁻², or 3.75.

All of these floating-point literals have a decimal value of 12.1875:

let decimalDouble = 12.1875
let exponentDouble = 1.21875e1
let hexadecimalDouble = 0xC.3p0

Numeric literals can contain extra formatting to make them easier to read. Both integers and floats can be padded with extra zeros and can contain underscores to help with readability. Neither type of formatting affects the underlying value of the literal:

let paddedDouble = 000123.456
let oneMillion = 1_000_000
let justOverOneMillion = 1_000_000.000_000_1

Numeric Type Conversion

Use the Int type for all general-purpose integer constants and variables in your code, even if they're known to be nonnegative. Using the default integer type in everyday situations means that integer constants and variables are immediately interoperable in your code and will match the inferred type for integer literal values.

Use other integer types only when they're specifically needed for the task at hand, because of explicitly sized data from an external source, or for performance, memory usage, or other necessary optimization. Using explicitly sized types in these situations helps to catch any accidental value overflows and implicitly documents the nature of the data being used.

Integer Conversion

The range of numbers that can be stored in an integer constant or variable is different for each numeric type. An Int8 constant or variable can store numbers between -128 and 127, whereas a UInt8 constant or variable can store numbers between 0 and 255. A number that won't fit into a constant or variable of a sized integer type is reported as an error when your code is compiled:

let cannotBeNegative: UInt8 = -1
// UInt8 can't store negative numbers, and so this will report an error
let tooBig: Int8 = Int8.max + 1
// Int8 can't store a number larger than its maximum value,
// and so this will also report an error

Because each numeric type can store a different range of values, you must opt in to numeric type conversion on a case-by-case basis. This opt-in approach prevents hidden conversion errors and helps make type conversion intentions explicit in your code.

To convert one specific number type to another, you initialize a new number of the desired type with the existing value. In the example below, the constant twoThousand is of type UInt16, whereas the constant one is of type UInt8. They can't be added together directly, because they're not of the same type. Instead, this example calls UInt16(one) to create a new UInt16 initialized with the value of one, and uses this value in place of the original:

let twoThousand: UInt16 = 2_000
let one: UInt8 = 1
let twoThousandAndOne = twoThousand + UInt16(one)

Because both sides of the addition are now of type UInt16, the addition is allowed. The output constant (twoThousandAndOne) is inferred to be of type UInt16, because it's the sum of two UInt16 values.

SomeType(ofInitialValue) is the default way to call the initializer of a Swift type and pass in an initial value. Behind the scenes, UInt16 has an initializer that accepts a UInt8 value, and so this initializer is used to make a new UInt16 from an existing UInt8. You can't pass in any type here, however --- it has to be a type for which UInt16 provides an initializer. Extending existing types to provide initializers that accept new types (including your own type definitions) is covered in Extensions.

Integer and Floating-Point Conversion

Conversions between integer and floating-point numeric types must be made explicit:

let three = 3
let pointOneFourOneFiveNine = 0.14159
let pi = Double(three) + pointOneFourOneFiveNine
// pi equals 3.14159, and is inferred to be of type Double

Here, the value of the constant three is used to create a new value of type Double, so that both sides of the addition are of the same type. Without this conversion in place, the addition would not be allowed.

Floating-point to integer conversion must also be made explicit. An integer type can be initialized with a Double or Float value:

let integerPi = Int(pi)
// integerPi equals 3, and is inferred to be of type Int

Floating-point values are always truncated when used to initialize a new integer value in this way. This means that 4.75 becomes 4, and -3.9 becomes -3.

Note: The rules for combining numeric constants and variables are different from the rules for numeric literals. The literal value 3 can be added directly to the literal value 0.14159, because number literals don't have an explicit type in and of themselves. Their type is inferred only at the point that they're evaluated by the compiler.

Type Aliases

Type aliases define an alternative name for an existing type. You define type aliases with the typealias keyword.

Type aliases are useful when you want to refer to an existing type by a name that's contextually more appropriate, such as when working with data of a specific size from an external source:

typealias AudioSample = UInt16

Once you define a type alias, you can use the alias anywhere you might use the original name:

var maxAmplitudeFound = AudioSample.min
// maxAmplitudeFound is now 0

Here, AudioSample is defined as an alias for UInt16. Because it's an alias, the call to AudioSample.min actually calls UInt16.min, which provides an initial value of 0 for the maxAmplitudeFound variable.

Booleans

Swift has a basic Boolean type, called Bool. Boolean values are referred to as logical, because they can only ever be true or false. Swift provides two Boolean constant values, true and false:

let orangesAreOrange = true
let turnipsAreDelicious = false

The types of orangesAreOrange and turnipsAreDelicious have been inferred as Bool from the fact that they were initialized with Boolean literal values. As with Int and Double above, you don't need to declare constants or variables as Bool if you set them to true or false as soon as you create them.

Boolean values are particularly useful when you work with conditional statements such as the if statement:

if turnipsAreDelicious {
    print("Mmm, tasty turnips!")
} else {
    print("Eww, turnips are horrible.")
}
// Prints "Eww, turnips are horrible."

Conditional statements such as the if statement are covered in more detail in ControlFlow.

Swift's type safety prevents non-Boolean values from being substituted for Bool. The following example reports a compile-time error:

let i = 1
if i {
    // this example will not compile, and will report an error
}

However, the alternative example below is valid:

let i = 1
if i == 1 {
    // this example will compile successfully
}

The result of the i == 1 comparison is of type Bool, and so this second example passes the type checking. Comparisons like i == 1 are discussed in BasicOperators.

As with other examples of type safety in Swift, this approach avoids accidental errors and ensures that the intention of a particular section of code is always clear.

Tuples

Tuples group multiple values into a single compound value. The values within a tuple can be of any type and don't have to be of the same type as each other.

In this example, (404, "Not Found") is a tuple that describes an HTTP status code. An HTTP status code is a special value returned by a web server whenever you request a web page. A status code of 404 Not Found is returned if you request a webpage that doesn't exist.

let http404Error = (404, "Not Found")
// http404Error is of type (Int, String), and equals (404, "Not Found")

The (404, "Not Found") tuple groups together an Int and a String to give the HTTP status code two separate values: a number and a human-readable description. It can be described as “a tuple of type (Int, String)”.

You can create tuples from any permutation of types, and they can contain as many different types as you like. There's nothing stopping you from having a tuple of type (Int, Int, Int), or (String, Bool), or indeed any other permutation you require.

You can decompose a tuple's contents into separate constants or variables, which you then access as usual:

let (statusCode, statusMessage) = http404Error
print("The status code is \(statusCode)")
// Prints "The status code is 404".
print("The status message is \(statusMessage)")
// Prints "The status message is Not Found".

If you only need some of the tuple's values, ignore parts of the tuple with an underscore (_) when you decompose the tuple:

let (justTheStatusCode, _) = http404Error
print("The status code is \(justTheStatusCode)")
// Prints "The status code is 404".

Alternatively, access the individual element values in a tuple using index numbers starting at zero:

print("The status code is \(http404Error.0)")
// Prints "The status code is 404".
print("The status message is \(http404Error.1)")
// Prints "The status message is Not Found".

You can name the individual elements in a tuple when the tuple is defined:

let http200Status = (statusCode: 200, description: "OK")

If you name the elements in a tuple, you can use the element names to access the values of those elements:

print("The status code is \(http200Status.statusCode)")
// Prints "The status code is 200".
print("The status message is \(http200Status.description)")
// Prints "The status message is OK".

Tuples are particularly useful as the return values of functions. A function that tries to retrieve a web page might return the (Int, String) tuple type to describe the success or failure of the page retrieval. By returning a tuple with two distinct values, each of a different type, the function provides more useful information about its outcome than if it could only return a single value of a single type. For more information, see Functions — Functions with Multiple Return Values.

Note: Tuples are useful for simple groups of related values. They're not suited to the creation of complex data structures. If your data structure is likely to be more complex, model it as a class or structure, rather than as a tuple. For more information, see ClassesAndStructures.

Optionals

You use *optionals* in situations where a value may be absent.
An optional represents two possibilities:
Either there *is* a value of a specified type,
and you can unwrap the optional to access that value,
or there *isn't* a value at all.

As an example of a value that might be missing,
Swift's `Int` type has an initializer
that tries to convert a `String` value into an `Int` value.
However, only some strings can be converted into integers.
The string `"123"` can be converted into the numeric value `123`,
but the string `"hello, world"` doesn't have a corresponding numeric value.
The example below uses the initializer to try to convert a `String` into an `Int`:

```swift
let possibleNumber = "123"
let convertedNumber = Int(possibleNumber)
// The type of convertedNumber is "optional Int"
```

<!--
  - test: `optionals`

  ```swifttest
  -> let possibleNumber = "123"
  -> let convertedNumber = Int(possibleNumber)
  // convertedNumber is inferred to be of type "Int?", or "optional Int"
  >> print(type(of: convertedNumber))
  << Optional<Int>
  ```
-->

Because the initializer in the code above might fail,
it returns an *optional* `Int`, rather than an `Int`.

To write an optional type,
you write a question mark (`?`)
after the name of the type that the optional contains ---
for example, the type of an optional `Int` is `Int?`.
An optional `Int` always contains
either some `Int` value or no value at all.
It can't contain anything else, like a `Bool` or `String` value.

### nil

You set an optional variable to a valueless state
by assigning it the special value `nil`:

```swift
var serverResponseCode: Int? = 404
// serverResponseCode contains an actual Int value of 404
serverResponseCode = nil
// serverResponseCode now contains no value
```

<!--
  - test: `optionals`

  ```swifttest
  -> var serverResponseCode: Int? = 404
  /> serverResponseCode contains an actual Int value of \(serverResponseCode!)
  </ serverResponseCode contains an actual Int value of 404
  -> serverResponseCode = nil
  // serverResponseCode now contains no value
  ```
-->

If you define an optional variable without providing a default value,
the variable is automatically set to `nil`:

```swift
var surveyAnswer: String?
// surveyAnswer is automatically set to nil
```

<!--
  - test: `optionals`

  ```swifttest
  -> var surveyAnswer: String?
  // surveyAnswer is automatically set to nil
  ```
-->

You can use an `if` statement to find out whether an optional contains a value
by comparing the optional against `nil`.
You perform this comparison with the “equal to” operator (`==`)
or the “not equal to” operator (`!=`).

If an optional has a value, it's considered as “not equal to” `nil`:

```swift
let possibleNumber = "123"
let convertedNumber = Int(possibleNumber)

if convertedNumber != nil {
    print("convertedNumber contains some integer value.")
}
// Prints "convertedNumber contains some integer value."
```

<!--
  - test: `optionals`

  ```swifttest
  -> if convertedNumber != nil {
        print("convertedNumber contains some integer value.")
     }
  <- convertedNumber contains some integer value.
  ```
-->

You can't use `nil` with non-optional constants or variables.
If a constant or variable in your code needs to work with
the absence of a value under certain conditions,
declare it as an optional value of the appropriate type.
A constant or variable that's declared as a non-optional value
is guaranteed to never contain a `nil` value.
If you try to assign `nil` to a non-optional value,
you'll get a compile-time error.

This separation of optional and non-optional values
lets you explicitly mark what information can be missing,
and makes it easier to write code that handle missing values.
You can't accidentally treat an optional as if it were non-optional
because this mistake produces an error at compile time.
After you unwrap the value,
none of the other code that works with that value needs to check for `nil`,
so there's no need to repeatedly check the same value
in different parts of your code.

When you access an optional value,
your code always handles both the `nil` and non-`nil` case.
There are several things you can do when a value is missing,
as described in the following sections:

- Skip the code that operates on the value when it's `nil`.

- Propagate the `nil` value,
  by returning `nil`
  or using the `?.` operator described in <doc:OptionalChaining>.

- Provide a fallback value, using the `??` operator.

- Stop program execution, using the `!` operator.

> Note:
> In Objective-C, `nil` is a pointer to a nonexistent object.
> In Swift, `nil` isn't a pointer --- it's the absence of a value of a certain type.
> Optionals of *any* type can be set to `nil`, not just object types.

### Optional Binding

You use optional binding to find out whether an optional contains a value,
and if so, to make that value available as a temporary constant or variable.
Optional binding can be used with `if`, `guard`, and `while` statements
to check for a value inside an optional,
and to extract that value into a constant or variable,
as part of a single action.
For more information about `if`, `guard`, and `while` statements,
see <doc:ControlFlow>.

Write an optional binding for an `if` statement as follows:

```swift
if let <#constantName#> = <#someOptional#> {
   <#statements#>
}
```

You can rewrite the `possibleNumber` example from
the <doc:TheBasics#Optionals> section
to use optional binding rather than forced unwrapping:

```swift
if let actualNumber = Int(possibleNumber) {
    print("The string \"\(possibleNumber)\" has an integer value of \(actualNumber)")
} else {
    print("The string \"\(possibleNumber)\" couldn't be converted to an integer")
}
// Prints "The string "123" has an integer value of 123".
```

<!--
  - test: `optionals`

  ```swifttest
  -> if let actualNumber = Int(possibleNumber) {
        print("The string \"\(possibleNumber)\" has an integer value of \(actualNumber)")
     } else {
        print("The string \"\(possibleNumber)\" couldn't be converted to an integer")
     }
  <- The string "123" has an integer value of 123
  ```
-->

This code can be read as:

“If the optional `Int` returned by `Int(possibleNumber)` contains a value,
set a new constant called `actualNumber` to the value contained in the optional.”

If the conversion is successful,
the `actualNumber` constant becomes available for use within
the first branch of the `if` statement.
It has already been initialized with the value contained within the optional,
and has the corresponding non-optional type.
In this case, the type of `possibleNumber` is `Int?`,
so the type of `actualNumber` is `Int`.

If you don't need to refer to the original, optional constant or variable
after accessing the value it contains,
you can use the same name for the new constant or variable:

```swift
let myNumber = Int(possibleNumber)
// Here, myNumber is an optional integer
if let myNumber = myNumber {
    // Here, myNumber is a non-optional integer
    print("My number is \(myNumber)")
}
// Prints "My number is 123".
```

<!--
  - test: `optionals`

  ```swifttest
  -> let myNumber = Int(possibleNumber)
  // Here, myNumber is an optional integer
  -> if let myNumber = myNumber {
         // Here, myNumber is a non-optional integer
         print("My number is \(myNumber)")
     }
  <- My number is 123
  ```
-->

This code starts by checking whether `myNumber` contains a value,
just like the code in the previous example.
If `myNumber` has a value,
the value of a new constant named `myNumber` is set to that value.
Inside the body of the `if` statement,
writing `myNumber` refers to that new non-optional constant.
Writing `myNumber` before or after the `if` statement
refers to the original optional integer constant.

Because this kind of code is so common,
you can use a shorter spelling to unwrap an optional value:
Write just the name of the constant or variable that you're unwrapping.
The new, unwrapped constant or variable
implicitly uses the same name as the optional value.

```swift
if let myNumber {
    print("My number is \(myNumber)")
}
// Prints "My number is 123".
```

<!--
  - test: `optionals`

  ```swifttest
  -> if let myNumber {
         print("My number is \(myNumber)")
     }
  <- My number is 123
  ```
-->

You can use both constants and variables with optional binding.
If you wanted to manipulate the value of `myNumber`
within the first branch of the `if` statement,
you could write `if var myNumber` instead,
and the value contained within the optional
would be made available as a variable rather than a constant.
Changes you make to `myNumber` inside the body of the `if` statement
apply only to that local variable,
*not* to the original, optional constant or variable that you unwrapped.

You can include as many optional bindings and Boolean conditions
in a single `if` statement as you need to,
separated by commas.
If any of the values in the optional bindings are `nil`
or any Boolean condition evaluates to `false`,
the whole `if` statement's condition
is considered to be `false`.
The following `if` statements are equivalent:

```swift
if let firstNumber = Int("4"), let secondNumber = Int("42"), firstNumber < secondNumber && secondNumber < 100 {
    print("\(firstNumber) < \(secondNumber) < 100")
}
// Prints "4 < 42 < 100".

if let firstNumber = Int("4") {
    if let secondNumber = Int("42") {
        if firstNumber < secondNumber && secondNumber < 100 {
            print("\(firstNumber) < \(secondNumber) < 100")
        }
    }
}
// Prints "4 < 42 < 100".
```

<!--
  - test: `multipleOptionalBindings`

  ```swifttest
  -> if let firstNumber = Int("4"), let secondNumber = Int("42"), firstNumber < secondNumber && secondNumber < 100 {
        print("\(firstNumber) < \(secondNumber) < 100")
     }
  <- 4 < 42 < 100

  -> if let firstNumber = Int("4") {
         if let secondNumber = Int("42") {
             if firstNumber < secondNumber && secondNumber < 100 {
                 print("\(firstNumber) < \(secondNumber) < 100")
             }
         }
     }
  <- 4 < 42 < 100
  ```
-->

<!--
  The example above uses multiple optional bindings
  to show that you can have more than one
  and to show the short-circuiting behavior.
  It has multiple Boolean conditions
  to show that you should join logically related conditions
  using the && operator instead of a comma.
-->

Constants and variables created with optional binding in an `if` statement
are available only within the body of the `if` statement.
In contrast, the constants and variables created with a `guard` statement
are available in the lines of code that follow the `guard` statement,
as described in <doc:ControlFlow#Early-Exit>.

### Providing a Fallback Value

Another way to handle a missing value is to supply
a default value using the nil-coalescing operator (`??`).
If the optional on the left of the `??` isn't `nil`,
that value is unwrapped and used.
Otherwise, the value on the right of `??` is used.
For example,
the code below greets someone by name if one is specified,
and uses a generic greeting when the name is `nil`.

```swift
let name: String? = nil
let greeting = "Hello, " + (name ?? "friend") + "!"
print(greeting)
// Prints "Hello, friend!"
```

<!--
.. testcode:: optionalFallback

   ```swifttest
   -> let name: String? = nil
   -> let greeting = "Hello, " + (name ?? "friend") + "!"
   -> print(greeting)
   <- Hello, friend!
   ```
-->

For more information about using `??` to provide a fallback value,
see <doc:BasicOperators#Nil-Coalescing-Operator>.

### Force Unwrapping

When `nil` represents an unrecoverable failure,
such as a programmer error or corrupted state,
you can access the underlying value of the optional
by adding an exclamation mark (`!`) to the end of the optional's name.
This is known as *force unwrapping* the optional's value.
When you force unwrap a non-`nil` value,
the result is its unwrapped value.
Force unwrapping a `nil` value triggers a runtime error.

The `!` is, effectively, a shorter spelling of [`fatalError(_:file:line:)`][].
For example, the code below shows two equivalent approaches:

[`fatalError(_:file:line:)`]: https://developer.apple.com/documentation/swift/fatalerror(_:file:line:)

```swift
let possibleNumber = "123"
let convertedNumber = Int(possibleNumber)

let number = convertedNumber!

guard let number = convertedNumber else {
    fatalError("The number was invalid")
}
```

Both versions of the code above depend on `convertedNumber`
always containing a value.
Writing that requirement as part of the code,
using either of the approaches above,
lets your code check that the requirement is true at runtime.

For more information about enforcing data requirements
and checking assumptions at runtime,
see <doc:TheBasics#Assertions-and-Preconditions>.

### Implicitly Unwrapped Optionals

As described above,
optionals indicate that a constant or variable is allowed to have “no value”.
Optionals can be checked with an `if` statement to see if a value exists,
and can be conditionally unwrapped with optional binding
to access the optional's value if it does exist.

Sometimes it's clear from a program's structure that an optional will *always* have a value,
after that value is first set.
In these cases, it's useful to remove the need
to check and unwrap the optional's value every time it's accessed,
because it can be safely assumed to have a value all of the time.

These kinds of optionals are defined as *implicitly unwrapped optionals*.
You write an implicitly unwrapped optional by placing an exclamation point (`String!`)
rather than a question mark (`String?`) after the type that you want to make optional.
Rather than placing an exclamation point after the optional's name when you use it,
you place an exclamation point after the optional's type when you declare it.

Implicitly unwrapped optionals are useful when
an optional's value is confirmed to exist immediately after the optional is first defined
and can definitely be assumed to exist at every point thereafter.
The primary use of implicitly unwrapped optionals in Swift is during class initialization,
as described in <doc:AutomaticReferenceCounting#Unowned-References-and-Implicitly-Unwrapped-Optional-Properties>.

Don't use an implicitly unwrapped optional when there's a possibility of
a variable becoming `nil` at a later point.
Always use a normal optional type if you need to check for a `nil` value
during the lifetime of a variable.

An implicitly unwrapped optional is a normal optional behind the scenes,
but can also be used like a non-optional value,
without the need to unwrap the optional value each time it's accessed.
The following example shows the difference in behavior between
an optional string and an implicitly unwrapped optional string
when accessing their wrapped value as an explicit `String`:

```swift
let possibleString: String? = "An optional string."
let forcedString: String = possibleString! // Requires explicit unwrapping

let assumedString: String! = "An implicitly unwrapped optional string."
let implicitString: String = assumedString // Unwrapped automatically
```

<!--
  - test: `implicitlyUnwrappedOptionals`

  ```swifttest
  -> let possibleString: String? = "An optional string."
  -> let forcedString: String = possibleString! // requires an exclamation point

  -> let assumedString: String! = "An implicitly unwrapped optional string."
  -> let implicitString: String = assumedString // no need for an exclamation point
  ```
-->

You can think of an implicitly unwrapped optional as
giving permission for the optional to be force-unwrapped if needed.
When you use an implicitly unwrapped optional value,
Swift first tries to use it as an ordinary optional value;
if it can't be used as an optional, Swift force-unwraps the value.
In the code above,
the optional value `assumedString` is force-unwrapped
before assigning its value to `implicitString`
because `implicitString` has an explicit, non-optional type of `String`.
In code below,
`optionalString` doesn't have an explicit type
so it's an ordinary optional.

```swift
let optionalString = assumedString
// The type of optionalString is "String?" and assumedString isn't force-unwrapped.
```

<!--
  - test: `implicitlyUnwrappedOptionals`

  ```swifttest
  -> let optionalString = assumedString
  // The type of optionalString is "String?" and assumedString isn't force-unwrapped.
  >> print(type(of: optionalString))
  << Optional<String>
  ```
-->

If an implicitly unwrapped optional is `nil` and you try to access its wrapped value,
you'll trigger a runtime error.
The result is exactly the same as if you write an exclamation point
to force unwrap a normal optional that doesn't contain a value.

You can check whether an implicitly unwrapped optional is `nil`
the same way you check a normal optional:

```swift
if assumedString != nil {
    print(assumedString!)
}
// Prints "An implicitly unwrapped optional string."
```

<!--
  - test: `implicitlyUnwrappedOptionals`

  ```swifttest
  -> if assumedString != nil {
        print(assumedString!)
     }
  <- An implicitly unwrapped optional string.
  ```
-->

You can also use an implicitly unwrapped optional with optional binding,
to check and unwrap its value in a single statement:

```swift
if let definiteString = assumedString {
    print(definiteString)
}
// Prints "An implicitly unwrapped optional string."
```

<!--
  - test: `implicitlyUnwrappedOptionals`

  ```swifttest
  -> if let definiteString = assumedString {
        print(definiteString)
     }
  <- An implicitly unwrapped optional string.
  ```
-->

Memory Safety

In addition to the checks that prevent type mismatches, described above in TheBasics — Type Safety and Type Inference, Swift also protects code against working with invalid memory. This protection is known as memory safety and includes the following requirements:

  • Values are set before being read.

The protection against interacting with uninitialized regions of memory is also known as definite initialization.

  • Arrays and buffers are accessed only at valid indexes.

The protection against out-of-bounds access is also known as bounds safety.

  • Memory is accessed only during the value’s lifetime.

The protection against use-after-free errors is also known as lifetime safety.

  • Access to memory overlaps only in provably safe ways.

The protection against possible data races in concurrent code is also known as thread safety.

If you've worked in languages that don't provide these guarantees, you may be familiar with some of the errors and bugs named in the list above. If you haven't encountered these issues, that's ok; safe code in Swift avoids these problems. For information about how Swift ensures you set initial values, see Initialization, for information about how Swift checks memory safety in concurrent code, see Concurrency, and for information about how Swift checks overlapping accesses to memory, see MemorySafety.

Sometimes you need to work outside of the bounds of safety --- for example, because of limitations of the language or standard library --- so Swift also provides unsafe versions of some APIs. When you use types or methods whose name includes words such as "unsafe", "unchecked", or "unmanaged", you take on the responsibility for safety.

Safe code in Swift can still encounter errors and unexpected failures, which might stop the program's execution. Safety doesn't ensure that your code runs to completion. Swift provides several ways to indicate and recover from errors, discussed in TheBasics — Error Handling and TheBasics — Assertions and Preconditions below. However, in some cases, the only safe way to handle an error is to stop execution. If you need to guarantee that a service never unexpectedly stops, incorporate fault tolerance into its overall architecture, so it can recover from any of its components stopping unexpectedly.

Error Handling

You use error handling to respond to error conditions your program may encounter during execution.

In contrast to optionals, which can use the presence or absence of a value to communicate success or failure of a function, error handling allows you to determine the underlying cause of failure, and, if necessary, propagate the error to another part of your program.

When a function encounters an error condition, it throws an error. That function's caller can then catch the error and respond appropriately.

func canThrowAnError() throws {
    // this function may or may not throw an error
}

A function indicates that it can throw an error by including the throws keyword in its declaration. When you call a function that can throw an error, you prepend the try keyword to the expression.

Swift automatically propagates errors out of their current scope until they're handled by a catch clause.

do {
    try canThrowAnError()
    // no error was thrown
} catch {
    // an error was thrown
}

A do statement creates a new containing scope, which allows errors to be propagated to one or more catch clauses.

Here's an example of how error handling can be used to respond to different error conditions:

func makeASandwich() throws {
    // ...
}

do {
    try makeASandwich()
    eatASandwich()
} catch SandwichError.outOfCleanDishes {
    washDishes()
} catch SandwichError.missingIngredients(let ingredients) {
    buyGroceries(ingredients)
}

In this example, the makeASandwich() function will throw an error if no clean dishes are available or if any ingredients are missing. Because makeASandwich() can throw an error, the function call is wrapped in a try expression. By wrapping the function call in a do statement, any errors that are thrown will be propagated to the provided catch clauses.

If no error is thrown, the eatASandwich() function is called. If an error is thrown and it matches the SandwichError.outOfCleanDishes case, then the washDishes() function will be called. If an error is thrown and it matches the SandwichError.missingIngredients case, then the buyGroceries(_:) function is called with the associated [String] value captured by the catch pattern.

Throwing, catching, and propagating errors is covered in greater detail in ErrorHandling.

Assertions and Preconditions

*Assertions* and *preconditions*
are checks that happen at runtime.
You use them to make sure an essential condition is satisfied
before executing any further code.
If the Boolean condition in the assertion or precondition
evaluates to `true`,
code execution continues as usual.
If the condition evaluates to `false`,
the current state of the program is invalid;
code execution ends, and your app is terminated.

You use assertions and preconditions
to express the assumptions you make
and the expectations you have
while coding,
so you can include them as part of your code.
Assertions help you find mistakes and incorrect assumptions during development,
and preconditions help you detect issues in production.

In addition to verifying your expectations at runtime,
assertions and preconditions also become a useful form of documentation
within the code.
Unlike the error conditions discussed in <doc:TheBasics#Error-Handling> above,
assertions and preconditions aren't used
for recoverable or expected errors.
Because a failed assertion or precondition
indicates an invalid program state,
there's no way to catch a failed assertion.
Recovering from an invalid state is impossible.
When an assertion fails,
at least one piece of the program's data is invalid ---
but you don't know why it's invalid
or whether an additional state is also invalid.

Using assertions and preconditions
isn't a substitute for designing your code in such a way
that invalid conditions are unlikely to arise.
However,
using them to enforce valid data and state
causes your app to terminate more predictably
if an invalid state occurs,
and helps make the problem easier to debug.
When assumptions aren't checked,
you might not notice this kind problem until much later
when code elsewhere starts failing visibly,
and after user data has been silently corrupted.
Stopping execution as soon as an invalid state is detected
also helps limit the damage caused by that invalid state.

The difference between assertions and preconditions is in when they're checked:
Assertions are checked only in debug builds,
but preconditions are checked in both debug and production builds.
In production builds,
the condition inside an assertion isn't evaluated.
This means you can use as many assertions as you want
during your development process,
without impacting performance in production.

### Debugging with Assertions

<!--
  If your code triggers an assertion while running in a debug environment,
  such as when you build and run an app in Xcode,
  you can see exactly where the invalid state occurred
  and query the state of your app at the time that the assertion was triggered.
  An assertion also lets you provide a suitable debug message as to the nature of the assert.
-->

You write an assertion by calling the
[`assert(_:_:file:line:)`](https://developer.apple.com/documentation/swift/1541112-assert) function
from the Swift standard library.
You pass this function an expression that evaluates to `true` or `false`
and a message to display if the result of the condition is `false`.
For example:

```swift
let age = -3
assert(age >= 0, "A person's age can't be less than zero.")
// This assertion fails because -3 isn't >= 0.
```

<!--
  - test: `assertions-1`

  ```swifttest
  -> let age = -3
  -> assert(age >= 0, "A person's age can't be less than zero.")
  xx assert
  // This assertion fails because -3 isn't >= 0.
  ```
-->

In this example, code execution continues if `age >= 0` evaluates to `true`,
that is, if the value of `age` is nonnegative.
If the value of `age` is negative, as in the code above,
then `age >= 0` evaluates to `false`,
and the assertion fails, terminating the application.

You can omit the assertion message ---
for example, when it would just repeat the condition as prose.

```swift
assert(age >= 0)
```

<!--
  - test: `assertions-2`

  ```swifttest
  >> let age = -3
  -> assert(age >= 0)
  xx assert
  ```
-->

<!--
  - test: `assertionsCanUseStringInterpolation`

  ```swifttest
  -> let age = -3
  -> assert(age >= 0, "A person's age can't be less than zero, but value is \(age).")
  xx assert
  ```
-->

If the code already checks the condition,
you use the
[`assertionFailure(_:file:line:)`](https://developer.apple.com/documentation/swift/1539616-assertionfailure) function
to indicate that an assertion has failed.
For example:

```swift
if age > 10 {
    print("You can ride the roller-coaster or the ferris wheel.")
} else if age >= 0 {
    print("You can ride the ferris wheel.")
} else {
    assertionFailure("A person's age can't be less than zero.")
}
```

<!--
  - test: `assertions-3`

  ```swifttest
  >> let age = -3
  -> if age > 10 {
         print("You can ride the roller-coaster or the ferris wheel.")
     } else if age >= 0 {
         print("You can ride the ferris wheel.")
     } else {
         assertionFailure("A person's age can't be less than zero.")
     }
  xx assert
  ```
-->

### Enforcing Preconditions

Use a precondition whenever a condition has the potential to be false,
but must *definitely* be true for your code to continue execution.
For example, use a precondition to check that a subscript isn't out of bounds,
or to check that a function has been passed a valid value.

You write a precondition by calling the
[`precondition(_:_:file:line:)`](https://developer.apple.com/documentation/swift/1540960-precondition) function.
You pass this function an expression that evaluates to `true` or `false`
and a message to display if the result of the condition is `false`.
For example:

```swift
// In the implementation of a subscript...
precondition(index > 0, "Index must be greater than zero.")
```

<!--
  - test: `preconditions`

  ```swifttest
  >> let index = -1
  // In the implementation of a subscript...
  -> precondition(index > 0, "Index must be greater than zero.")
  xx assert
  ```
-->

You can also call the
[`preconditionFailure(_:file:line:)`](https://developer.apple.com/documentation/swift/1539374-preconditionfailure) function
to indicate that a failure has occurred ---
for example, if the default case of a switch was taken,
but all valid input data should have been handled
by one of the switch's other cases.

> Note: If you compile in unchecked mode (`-Ounchecked`),
> preconditions aren't checked.
> The compiler assumes that preconditions are always true,
> and it optimizes your code accordingly.
> However, the `fatalError(_:file:line:)` function always halts execution,
> regardless of optimization settings.
>
> You can use the `fatalError(_:file:line:)` function
> during prototyping and early development
> to create stubs for functionality that hasn't been implemented yet,
> by writing `fatalError("Unimplemented")` as the stub implementation.
> Because fatal errors are never optimized out,
> unlike assertions or preconditions,
> you can be sure that execution always halts
> if it encounters a stub implementation.

<!--
  "\ " in the first cell below lets it be empty.
  Otherwise RST treats the row as a continuation.

  ============ =====  ==========  ===============================
  \            Debug  Production  Production with ``-Ounchecked``
  ============ =====  ==========  ===============================
  Assertion    Yes    No          No
  ------------ -----  ----------  -------------------------------
  Precondition Yes    Yes         No
  ------------ -----  ----------  -------------------------------
  Fatal Error  Yes    Yes         Yes
  ============ =====  ==========  ===============================
-->

<!--
  TODO: In Xcode, can you set a breakpoint on assertion/precondition failure?
  If so, mention that fact and give a link to a guide that shows you how.
  In LLDB, 'breakpoint set -E swift' catches when errors are thrown,
  but doesn't stop at assertions.
-->

<!--
This source file is part of the Swift.org open source project

Copyright (c) 2014 - 2022 Apple Inc. and the Swift project authors
Licensed under Apache License v2.0 with Runtime Library Exception

See https://swift.org/LICENSE.txt for license information
See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
-->