# Generics

> Version: *Swift 5.5*\
> Source: [*swift-book: Generics*](https://docs.swift.org/swift-book/LanguageGuide/Generics.html)\
> Digest Date: *February 1, 2022*

*Generic* code enables you to write flexible, reusable functions and types that can work with any type, subject to requirements that you define. You can write code that avoids duplication and expresses its intent in a clear, abstracted manner.

* [Generics](#generics)
  * [The Problem That Generics Solve](#the-problem-that-generics-solve)
  * [Generic Functions](#generic-functions)
  * [Type Parameters](#type-parameters)
  * [Naming Type Parameters](#naming-type-parameters)
  * [Generic Types](#generic-types)
  * [Extending a Generic Type](#extending-a-generic-type)
  * [Type Constraints](#type-constraints)
    * [Type Constraint Syntax](#type-constraint-syntax)
    * [Type Constraints in Action](#type-constraints-in-action)
  * [Associated Types](#associated-types)
    * [Associated Types in Action](#associated-types-in-action)
    * [Extending an Existing Type to Specify an Associated Type](#extending-an-existing-type-to-specify-an-associated-type)
    * [Adding Constraints to an Associated Type](#adding-constraints-to-an-associated-type)
    * [Using a Protocol in Its Associated Type’s Constraints](#using-a-protocol-in-its-associated-types-constraints)
  * [Generic Where Clauses](#generic-where-clauses)
  * [Extensions with a Generic Where Clause](#extensions-with-a-generic-where-clause)
  * [Contextual Where Clauses](#contextual-where-clauses)
  * [Associated Types with a Generic Where Clause](#associated-types-with-a-generic-where-clause)
  * [Generic Subscripts](#generic-subscripts)

## The Problem That Generics Solve

Here’s a standard, nongeneric function called `swapTwoInts(_:_:)`, which swaps two `Int` values:

```swift
func swapTwoInts(_ a: inout Int, _ b: inout Int) {
    let temporaryA = a
    a = b
    b = temporaryA
}
```

This function makes use of in-out parameters to swap the values of `a` and `b`, as described in [In-Out Parameters](https://docs.swift.org/swift-book/LanguageGuide/Functions.html#ID173).

The `swapTwoInts(_:_:)` function swaps the original value of `b` into `a`, and the original value of `a` into `b`. You can call this function to swap the values in two `Int` variables:

```swift
var someInt = 3
var anotherInt = 107
swapTwoInts(&someInt, &anotherInt)
print("someInt is now \(someInt), and anotherInt is now \(anotherInt)")
// Prints "someInt is now 107, and anotherInt is now 3"
```

The `swapTwoInts(_:_:`) function is useful, but it can only be used with `Int` values. If you want to swap two `String` values, or two `Double` values, you have to write more functions, such as the `swapTwoStrings(_:_:)` and `swapTwoDoubles(_:_:)` functions shown below:

```swift
func swapTwoStrings(_ a: inout String, _ b: inout String) {
    let temporaryA = a
    a = b
    b = temporaryA
}

func swapTwoDoubles(_ a: inout Double, _ b: inout Double) {
    let temporaryA = a
    a = b
    b = temporaryA
}
```

You may have noticed that the bodies of the `swapTwoInts(_:_:)`, `swapTwoStrings(_:_:)`, and `swapTwoDoubles(_:_:)` functions are identical. The only difference is the type of the values that they accept (`Int`, `String`, and `Double`).

It’s more useful, and considerably more flexible, to write a single function that swaps two values of *any* type. Generic code enables you to write such a function. (A generic version of these functions is defined below.)

## Generic Functions

Generic functions can work with any type. Here’s a generic version of the `swapTwoInts(_:_:)` function from above, called `swapTwoValues(_:_:)`:

```swift
func swapTwoValues<T>(_ a: inout T, _ b: inout T) {
    let temporaryA = a
    a = b
    b = temporaryA
}
```

The body of the `swapTwoValues(_:_:)` function is identical to the body of the `swapTwoInts(_:_:)` function. However, the first line of swapTwoValues(*:*:) is slightly different from `swapTwoInts(_:_:)`. Here’s how the first lines compare:

```swift
func swapTwoInts(_ a: inout Int, _ b: inout Int)
func swapTwoValues<T>(_ a: inout T, _ b: inout T)
```

The generic version of the function uses a *placeholder* type name (called `T`, in this case) instead of an *actual type* name (such as `Int`, `String`, or `Double`). The placeholder type name doesn’t say anything about what `T` must be, but it does say that both `a` and `b` must be of the same type `T`, whatever `T` represents. The actual type to use in place of `T` is determined each time the `swapTwoValues(_:_:)` function is called.

Each time `swapTwoValues(_:_:)` is called, the type to use for `T` is *inferred* from the types of values passed to the function.

In the two examples below, `T` is inferred to be `Int` and `String` respectively:

```swift
var someInt = 3
var anotherInt = 107
swapTwoValues(&someInt, &anotherInt)
// someInt is now 107, and anotherInt is now 3

var someString = "hello"
var anotherString = "world"
swapTwoValues(&someString, &anotherString)
// someString is now "world", and anotherString is now "hello"
```

> **NOTE**: The `swapTwoValues(_:_:)` function defined above is inspired by a generic function called `swap`, which is part of the Swift standard library, and is automatically made available for you to use in your apps. If you need the behavior of the `swapTwoValues(_:_:)` function in your own code, you can use Swift’s existing `swap(_:_:)` function rather than providing your own implementation.

## Type Parameters

In the `swapTwoValues(_:_:)` example above, the placeholder type `T` is an example of a *type parameter*. Type parameters specify and name a placeholder type, and are written immediately after the function’s name, between *a pair of matching angle brackets* (such as `<T>`).

You can provide more than one type parameter by writing multiple type parameter names within the angle brackets, separated by commas.

## Naming Type Parameters

In most cases, type parameters have descriptive names, such as `Key` and `Value` in `Dictionary<Key, Value>` and `Element` in `Array<Element>`, which tells the reader about the relationship between the type parameter and the generic type or function it’s used in.

However, when there isn’t a meaningful relationship between them, it’s traditional to name them using single letters such as `T`, `U`, and `V`, such as `T` in the `swapTwoValues(_:_:)` function above.

> **NOTE**: Always give type parameters *upper camel case* names (such as `T` and `MyTypeParameter`) to indicate that they’re a placeholder for a type, not a value.

## Generic Types

In addition to generic functions, Swift enables you to define your own *generic types*. These are custom *classes*, *structures*, and *enumerations* that can work with any type, in a similar way to `Array` and `Dictionary`.

This section shows you how to write a generic collection type called `Stack`.

Here’s how to write a nongeneric version of a stack, in this case for a stack of `Int` values:

```swift
struct IntStack {
    var items: [Int] = []
    mutating func push(_ item: Int) {
        items.append(item)
    }
    mutating func pop() -> Int {
        return items.removeLast()
    }
}
```

This structure uses an `Array` property called `items` to store the values in the stack. `Stack` provides two methods, `push` and `pop`, to push and pop values on and off the stack. These methods are marked as `mutating`, because they need to modify (or *mutate*) the structure’s `items` array.

The `IntStack` type shown above can only be used with `Int` values, however. It would be much more useful to define a *generic* `Stack` structure, that can manage a stack of *any* type of value.

Here’s a generic version of the same code:

```swift
struct Stack<Element> {
    var items: [Element] = []
    mutating func push(_ item: Element) {
        items.append(item)
    }
    mutating func pop() -> Element {
        return items.removeLast()
    }
}
```

Because it’s a generic type, `Stack` can be used to create a stack of *any* valid type in Swift, in a similar manner to `Array` and `Dictionary`.

You create a new `Stack` instance by writing the type to be stored in the stack within angle brackets. For example, to create a new stack of strings, you write `Stack<String>()`:

```swift
var stackOfStrings = Stack<String>()
stackOfStrings.push("uno")
stackOfStrings.push("dos")
stackOfStrings.push("tres")
stackOfStrings.push("cuatro")
// the stack now contains 4 strings
```

Popping a value from the stack removes and returns the top value, "`cuatro`":

```swift
let fromTheTop = stackOfStrings.pop()
// fromTheTop is equal to "cuatro", and the stack now contains 3 strings
```

## Extending a Generic Type

When you extend a generic type, you don’t provide a type parameter list as part of the extension’s definition. Instead, the type parameter list from the *original* type definition is available within the body of the extension, and the original type parameter names are used to refer to the type parameters from the original definition.

The following example extends the generic `Stack` type to add a read-only *computed property* called `topItem`, which returns the top item on the stack without popping it from the stack:

```swift
extension Stack {
    var topItem: Element? {
        return items.isEmpty ? nil : items[items.count - 1]
    }
}
```

The `topItem` computed property can now be used with any `Stack` instance to access and query its top item without removing it.

```swift
if let topItem = stackOfStrings.topItem {
    print("The top item on the stack is \(topItem).")
}
// Prints "The top item on the stack is tres."
```

Extensions of a generic type can also include requirements that instances of the extended type must satisfy in order to gain the new functionality, as discussed in [Extensions with a Generic Where Clause](#extensions-with-a-generic-where-clause) below.

## Type Constraints

The `swapTwoValues(_:_:)` function and the `Stack` type can work with any type. However, it’s sometimes useful to enforce certain *type constraints* on the types that can be used with generic functions and generic types. Type constraints specify that a type parameter must inherit from a specific class, or conform to a particular protocol or protocol composition.

For example, Swift’s `Dictionary` type places a limitation on the types that can be used as keys for a dictionary. As described in [Dictionaries](https://docs.swift.org/swift-book/LanguageGuide/CollectionTypes.html#ID113), the type of a dictionary’s keys must be *hashable*. That is, it must provide a way to make itself *uniquely* representable.

This requirement is enforced by a type constraint on the key type for `Dictionary`, which specifies that the key type must conform to the `Hashable` protocol, a special protocol defined in the Swift standard library. All of Swift’s *basic types* (such as `String`, `Int`, `Double`, and `Bool`) are hashable by default. For information about making your own custom types conform to the Hashable protocol, see [Conforming to the Hashable Protocol](https://developer.apple.com/documentation/swift/hashable#2849490).

You can define your own type constraints when creating custom generic types, and these constraints provide much of the power of generic programming. Abstract concepts like Hashable characterize types in terms of their conceptual characteristics, rather than their concrete type.

### Type Constraint Syntax

You write type constraints by placing a single class or protocol constraint after a type parameter’s name, separated by a *colon*, as part of the type parameter list.

The basic syntax for type constraints on a generic function is shown below (although the syntax is the same for generic types):

```swift
func someFunction<T: SomeClass, U: SomeProtocol>(someT: T, someU: U) {
    // function body goes here
}
```

### Type Constraints in Action

Here’s a nongeneric function called `findIndex(ofString:in:)`, which is given a `String` value to find and an array of `String` values within which to find it. The `findIndex(ofString:in:)` function returns an optional `Int` value, which will be the index of the first matching string in the array if it’s found, or `nil` if the string can’t be found:

```swift
func findIndex(ofString valueToFind: String, in array: [String]) -> Int? {
    for (index, value) in array.enumerated() {
        if value == valueToFind {
            return index
        }
    }
    return nil
}
```

The `findIndex(ofString:in:)` function can be used to find a string value in an array of strings:

```swift
let strings = ["cat", "dog", "llama", "parakeet", "terrapin"]
if let foundIndex = findIndex(ofString: "llama", in: strings) {
    print("The index of llama is \(foundIndex)")
}
// Prints "The index of llama is 2"
```

The principle of finding the index of a value in an array isn’t useful only for strings, however. You can write the same functionality as a generic function by replacing any mention of strings with values of some type `T` instead.

Here’s how you might expect a generic version of `findIndex(ofString:in:)`, called `findIndex(of:in:)`, to be written. Note that the return type of this function is still `Int?`, because the function returns an optional index number, not an optional value from the array. Be warned, though, this function doesn’t compile, for reasons explained after the example:

```swift
func findIndex<T>(of valueToFind: T, in array:[T]) -> Int? {
    for (index, value) in array.enumerated() {
        if value == valueToFind {
            return index
        }
    }
    return nil
}
```

This function doesn’t compile as written above. The problem lies with the equality check, “`if value == valueToFind`”. Not every type in Swift can be compared with the equal to operator (`==`). If you create your own class or structure to represent a complex data model, for example, then the meaning of “equal to” for that class or structure isn’t something that Swift can guess for you. Because of this, it isn’t possible to guarantee that this code will work for *every* possible type `T`, and an appropriate error is reported when you try to compile the code.

All is not lost, however. The Swift standard library defines a protocol called `Equatable`, which requires any conforming type to implement the equal to operator (`==`) and the not equal to operator (`!=`) to compare any two values of that type. All of Swift’s standard types automatically support the `Equatable` protocol.

Any type that’s `Equatable` can be used safely with the `findIndex(of:in:)` function, because it’s guaranteed to support the equal to operator. To express this fact, you write a type constraint of `Equatable` as part of the type parameter’s definition when you define the function:

```swift
func findIndex<T: Equatable>(of valueToFind: T, in array:[T]) -> Int? {
    for (index, value) in array.enumerated() {
        if value == valueToFind {
            return index
        }
    }
    return nil
}
```

The single type parameter for `findIndex(of:in:)` is written as `T: Equatable`, which means “any type `T` that conforms to the `Equatable` protocol.”

The `findIndex(of:in:)` function now compiles successfully and can be used with any type that’s `Equatable`, such as `Double` or `String`:

```swift
let doubleIndex = findIndex(of: 9.3, in: [3.14159, 0.1, 0.25])
// doubleIndex is an optional Int with no value, because 9.3 isn't in the array
let stringIndex = findIndex(of: "Andrea", in: ["Mike", "Malcolm", "Andrea"])
// stringIndex is an optional Int containing a value of 2
```

## Associated Types

When defining a protocol, it’s sometimes useful to declare one or more *associated types* as part of the protocol’s definition. An associated type gives a placeholder name to a type that’s used as part of the protocol. The actual type to use for that associated type isn’t specified until the protocol is adopted. Associated types are specified with the `associatedtype` keyword.

### Associated Types in Action

Here’s an example of a protocol called `Container`, which declares an associated type called `Item`:

```swift
protocol Container {
    associatedtype Item
    mutating func append(_ item: Item)
    var count: Int { get }
    subscript(i: Int) -> Item { get }
}
```

This protocol doesn’t specify how the items in the container should be stored or what type they’re allowed to be. The protocol only specifies the three bits of functionality that any type must provide in order to be considered a `Container`. A conforming type can provide additional functionality, as long as it satisfies these three requirements.

The `Container` protocol declares an associated type called `Item`, written as `associatedtype Item`. The protocol doesn’t define what Item is, that information is left for any conforming type to provide.

Nonetheless, the `Item` alias provides a way to refer to the type of the items in a Container, and to define a type for use with the `append(_:)` method and *subscript*, to ensure that the expected behavior of any `Container` is enforced.

Here’s a version of the nongeneric `IntStack` type from [Generic Types](#generic-types) above, adapted to conform to the `Container` protocol:

```swift
struct IntStack: Container {
    // original IntStack implementation
    var items: [Int] = []
    mutating func push(_ item: Int) {
        items.append(item)
    }
    mutating func pop() -> Int {
        return items.removeLast()
    }
    // conformance to the Container protocol
    typealias Item = Int
    mutating func append(_ item: Int) {
        self.push(item)
    }
    var count: Int {
        return items.count
    }
    subscript(i: Int) -> Int {
        return items[i]
    }
}
```

`IntStack` specifies that for this implementation of `Container`, the appropriate Item to use is a type of `Int`. The definition of `typealias Item = Int` turns the abstract type of `Item` into a concrete type of `Int` for this implementation of the `Container` protocol.

Thanks to Swift’s *type inference*, you don’t actually need to declare a concrete `Item` of `Int` as part of the definition of `IntStack`. Because `IntStack` conforms to all of the requirements of the `Container` protocol, Swift can *infer* the appropriate `Item` to use, simply by looking at the type of the `append(_:)` method’s `item` parameter and the return type of the subscript. *Indeed, if you delete the `typealias Item = Int` line from the code above, everything still works, because it’s clear what type should be used for Item.*

You can also make the generic `Stack` type conform to the `Container` protocol:

```swift
struct Stack<Element>: Container {
    // original Stack<Element> implementation
    var items: [Element] = []
    mutating func push(_ item: Element) {
        items.append(item)
    }
    mutating func pop() -> Element {
        return items.removeLast()
    }
    // conformance to the Container protocol
    mutating func append(_ item: Element) {
        self.push(item)
    }
    var count: Int {
        return items.count
    }
    subscript(i: Int) -> Element {
        return items[i]
    }
}
```

This time, the type parameter `Element` is used as the type of the `append(_:)` method’s `item` parameter and the return type of the subscript. Swift can therefore infer that Element is the appropriate type to use as the `Item` for this particular container.

### Extending an Existing Type to Specify an Associated Type

You can extend an existing type to add conformance to a protocol, as described in [Adding Protocol Conformance with an Extension](https://docs.swift.org/swift-book/LanguageGuide/Protocols.html#ID277). This includes a protocol with an *associated type*.

Swift’s `Array` type already provides an `append(_:)` method, a `count` property, and a subscript with an `Int` index to retrieve its elements. These three capabilities match the requirements of the `Container` protocol. This means that you can extend `Array` to conform to the Container protocol simply by declaring that `Array` adopts the protocol. You do this with an empty extension, as described in [Declaring Protocol Adoption with an Extension](https://docs.swift.org/swift-book/LanguageGuide/Protocols.html#ID278):

```swift
extension Array: Container {}
```

Array’s existing `append(_:)` method and subscript enable Swift to infer the appropriate type to use for `Item`, just as for the generic `Stack` type above. After defining this extension, you can use any `Array` as a `Container`.

### Adding Constraints to an Associated Type

You can add *type constraints* to an associated type in a protocol to require that conforming types satisfy those constraints.

For example, the following code defines a version of `Container` that requires the items in the container to be equatable.

```swift
protocol Container {
    associatedtype Item: Equatable
    mutating func append(_ item: Item)
    var count: Int { get }
    subscript(i: Int) -> Item { get }
}
```

### Using a Protocol in Its Associated Type’s Constraints

A protocol can appear as part of its own requirements. For example, here’s a protocol that refines the `Container` protocol, adding the requirement of a `suffix(_:)` method. The `suffix(_:)` method returns a given number of elements from the end of the container, storing them in an instance of the `Suffix` type.

```swift
protocol SuffixableContainer: Container {
    associatedtype Suffix: SuffixableContainer where Suffix.Item == Item
    func suffix(_ size: Int) -> Suffix
}
```

In this protocol, `Suffix` is an associated type, like the `Item` type in the `Container` example above. `Suffix` has two constraints:

* It must conform to the `SuffixableContainer` protocol (the protocol currently being defined),
* and its `Item` type must be the same as the container’s `Item` type.

The constraint on `Item` is a generic `where` clause, which is discussed in [Associated Types with a Generic Where Clause](#associated-types-with-a-generic-where-clause) below.

Here’s an extension of the `Stack` type from [Generic Types](#generic-types) above that adds conformance to the `SuffixableContainer` protocol:

```swift
extension Stack: SuffixableContainer {
    func suffix(_ size: Int) -> Stack {
        var result = Stack()
        for index in (count-size)..<count {
            result.append(self[index])
        }
        return result
    }
    // Inferred that Suffix is Stack.
}
var stackOfInts = Stack<Int>()
stackOfInts.append(10)
stackOfInts.append(20)
stackOfInts.append(30)
let suffix = stackOfInts.suffix(2)
// suffix contains 20 and 30
```

In the example above, the `Suffix` associated type for `Stack` is also `Stack`, so the suffix operation on `Stack` returns another `Stack`.

Alternatively, a type that conforms to `SuffixableContainer` can have a `Suffix` type that’s different from itself, meaning the suffix operation can return a different type.

For example, here’s an extension to the *nongeneric* `IntStack` type that adds `SuffixableContainer` conformance, using `Stack<Int>` as its suffix type instead of `IntStack`:

```swift
extension IntStack: SuffixableContainer {
    func suffix(_ size: Int) -> Stack<Int> {
        var result = Stack<Int>()
        for index in (count-size)..<count {
            result.append(self[index])
        }
        return result
    }
    // Inferred that Suffix is Stack<Int>.
}
```

## Generic Where Clauses

It can also be useful to define requirements for associated types. You do this by defining *a generic where clause*. A generic `where` clause enables you to require that an associated type must conform to a certain protocol, or that certain type parameters and associated types must be the same. A generic `where` clause starts with the `where` keyword, followed by constraints for associated types or equality relationships between types and associated types. You write a generic `where` clause right before the opening curly brace of a type or function’s body.

The example below defines a generic function called `allItemsMatch`, which checks to see if two `Container` instances contain the same items in the same order. The function returns a `Boolean` value of `true` if all items match and a value of `false` if they don’t.

The two containers to be checked don’t have to be the same type of container (although they can be), but they do have to hold the same type of items. This requirement is expressed through a combination of type constraints and a generic `where` clause:

```swift
func allItemsMatch<C1: Container, C2: Container>
    (_ someContainer: C1, _ anotherContainer: C2) -> Bool
    where C1.Item == C2.Item, C1.Item: Equatable {

        // Check that both containers contain the same number of items.
        if someContainer.count != anotherContainer.count {
            return false
        }

        // Check each pair of items to see if they're equivalent.
        for i in 0..<someContainer.count {
            if someContainer[i] != anotherContainer[i] {
                return false
            }
        }

        // All items match, so return true.
        return true
}
```

This function takes two arguments called `someContainer` and `anotherContainer`. The `someContainer` argument is of type `C1`, and the `anotherContainer` argument is of type `C2`. Both `C1` and `C2` are type parameters for two container types to be determined when the function is called.

The following requirements are placed on the function’s two type parameters:

* `C1` must conform to the `Container` protocol (written as `C1: Container`).
* `C2` must also conform to the `Container` protocol (written as `C2: Container`).
* The `Item` for `C1` must be the same as the `Item` for `C2` (written as `C1.Item == C2.Item`).
* The `Item` for `C1` must conform to the `Equatable` protocol (written as `C1.Item: Equatable`).

These requirements mean:

* `someContainer` is a container of type `C1`.
* `anotherContainer` is a container of type `C2`.
* `someContainer` and `anotherContainer` contain the same type of items.
* The items in `someContainer` can be checked with the not equal operator (`!=`) to see if they’re different from each other.

These requirements enable the `allItemsMatch(_:_:)` function to compare the two containers, even if they’re of a different container type.

Here’s how the `allItemsMatch(_:_:)` function looks in action:

```swift
var stackOfStrings = Stack<String>()
stackOfStrings.push("uno")
stackOfStrings.push("dos")
stackOfStrings.push("tres")

var arrayOfStrings = ["uno", "dos", "tres"]

if allItemsMatch(stackOfStrings, arrayOfStrings) {
    print("All items match.")
} else {
    print("Not all items match.")
}
// Prints "All items match."
```

Even though the stack and the array are of a different type, they both conform to the `Container` protocol, and both contain the same type of values. You can therefore call the `allItemsMatch(_:_:)` function with these two containers as its arguments.

In the example above, the `allItemsMatch(_:_:)` function correctly reports that all of the items in the two containers match.

## Extensions with a Generic Where Clause

You can also use a generic `where` clause as part of an extension. The example below extends the generic `Stack` structure from the previous examples to add an `isTop(_:)` method.

```swift
extension Stack where Element: Equatable {
    func isTop(_ item: Element) -> Bool {
        guard let topItem = items.last else {
            return false
        }
        return topItem == item
    }
}
```

This new `isTop(_:)` method first checks that the stack isn’t empty, and then compares the given item against the stack’s topmost item. If you tried to do this without a generic where clause, you would have a problem: The implementation of `isTop(_:)` uses the `==` operator, but the definition of Stack doesn’t require its items to be equatable, so using the `==` operator results in a compile-time error.

Using a generic `where` clause lets you add a new requirement to the extension, so that the extension adds the `isTop(_:)` method only when the items in the stack are equatable.

Here’s how the `isTop(_:)` method looks in action:

```swift
if stackOfStrings.isTop("tres") {
    print("Top element is tres.")
} else {
    print("Top element is something else.")
}
// Prints "Top element is tres."
```

If you try to call the `isTop(_:)` method on a stack whose elements aren’t equatable, you’ll get a compile-time error.

```swift
struct NotEquatable { }
var notEquatableStack = Stack<NotEquatable>()
let notEquatableValue = NotEquatable()
notEquatableStack.push(notEquatableValue)
notEquatableStack.isTop(notEquatableValue)  // Error
```

You can use a generic `where` clause with extensions to a protocol. The example below extends the `Container` protocol from the previous examples to add a `startsWith(_:)` method.

```swift
extension Container where Item: Equatable {
    func startsWith(_ item: Item) -> Bool {
        return count >= 1 && self[0] == item
    }
}
```

The `startsWith(_:)` method first makes sure that the container has at least one item, and then it checks whether the first item in the container matches the given item.

This new `startsWith(_:)` method can be used with any type that conforms to the `Container` protocol, including the stacks and arrays used above, as long as the container’s items are equatable.

```swift
if [9, 9, 9].startsWith(42) {
    print("Starts with 42.")
} else {
    print("Starts with something else.")
}
// Prints "Starts with something else."
```

The generic `where` clause in the example above requires `Item` to conform to a protocol, but you can also write a generic `where` clauses that require `Item` to be a specific type. For example:

```swift
extension Container where Item == Double {
    func average() -> Double {
        var sum = 0.0
        for index in 0..<count {
            sum += self[index]
        }
        return sum / Double(count)
    }
}
print([1260.0, 1200.0, 98.6, 37.0].average())
// Prints "648.9"
```

You can include multiple requirements in a generic `where` clause that’s part of an extension, just like you can for a generic `where` clause that you write elsewhere. Separate each requirement in the list with a *comma*.

## Contextual Where Clauses

You can write a generic `where` clause as part of a declaration that doesn’t have its own generic type constraints, when you’re already working in the context of generic types.

For example, you can write a generic `where` clause on a subscript of a generic type or on a method in an extension to a generic type. The `Container` structure is generic, and the `where` clauses in the example below specify what *type constraints* have to be satisfied to make these new methods available on a container.

```swift
extension Container {
    func average() -> Double where Item == Int {
        var sum = 0.0
        for index in 0..<count {
            sum += Double(self[index])
        }
        return sum / Double(count)
    }
    func endsWith(_ item: Item) -> Bool where Item: Equatable {
        return count >= 1 && self[count-1] == item
    }
}
let numbers = [1260, 1200, 98, 37]
print(numbers.average())
// Prints "648.75"
print(numbers.endsWith(37))
// Prints "true"
```

This example adds an `average()` method to `Container` when the items are integers, and it adds an `endsWith(_:)` method when the items are equatable. Both functions include a generic `where` clause that adds type constraints to the generic `Item` type parameter from the original declaration of `Container`.

If you want to write this code without using contextual `where` clauses, you write two extensions, one for each generic `where` clause. The example above and the example below have the same behavior.

```swift
extension Container where Item == Int {
    func average() -> Double {
        var sum = 0.0
        for index in 0..<count {
            sum += Double(self[index])
        }
        return sum / Double(count)
    }
}
extension Container where Item: Equatable {
    func endsWith(_ item: Item) -> Bool {
        return count >= 1 && self[count-1] == item
    }
}
```

In the version of this example that uses contextual `where` clauses, the implementation of `average()` and `endsWith(_:)` are both in the same extension because each method’s generic where clause states the requirements that need to be satisfied to make that method available.

Moving those requirements to the extensions’ generic `where` clauses makes the methods available in the same situations, but requires one extension per requirement.

## Associated Types with a Generic Where Clause

You can include a generic `where` clause on an associated type. For example, suppose you want to make a version of `Container` that includes an iterator, like what the `Sequence` protocol uses in the standard library. Here’s how you write that:

```swift
protocol Container {
    associatedtype Item
    mutating func append(_ item: Item)
    var count: Int { get }
    subscript(i: Int) -> Item { get }

    associatedtype Iterator: IteratorProtocol where Iterator.Element == Item
    func makeIterator() -> Iterator
}
```

The generic `where` clause on `Iterator` requires that the iterator must traverse over elements of the same item type as the container’s items, regardless of the iterator’s type. The `makeIterator()` function provides access to a container’s iterator.

For a protocol that inherits from another protocol, you add a constraint to an inherited associated type by including the generic `where` clause in the protocol declaration.

For example, the following code declares a `ComparableContainer` protocol that requires `Item` to conform to `Comparable`:

```swift
protocol ComparableContainer: Container where Item: Comparable { }
```

## Generic Subscripts

Subscripts can be generic, and they can include generic `where` clauses. You write the placeholder type name inside angle brackets after `subscript`, and you write a generic `where` clause right before the opening curly brace of the subscript’s body. For example:

```swift
extension Container {
    subscript<Indices: Sequence>(indices: Indices) -> [Item]
        where Indices.Iterator.Element == Int {
            var result: [Item] = []
            for index in indices {
                result.append(self[index])
            }
            return result
    }
}
```

This extension to the `Container` protocol adds a subscript that takes a sequence of indices and returns an array containing the items at each given index. This generic subscript is constrained as follows:

* The generic parameter `Indices` in angle brackets has to be a type that conforms to the `Sequence` protocol from the standard library.
* The subscript takes a single parameter, `indices`, which is an instance of that `Indices` type.
* The generic `where` clause requires that the iterator for the sequence must traverse over elements of type `Int`. This ensures that the indices in the sequence are the same type as the indices used for a container.

Taken together, these constraints mean that the value passed for the indices parameter is a sequence of integers.