the-swift-rogramming-language

return y

}

return x

}

Because Int and Double, for example, both conform to the Comparable protocol, this function accepts arguments of either type. In contrast with generic types, you don’t specify a generic argument clause when you use a generic function or initializer. The type arguments are instead inferred from the type of the arguments passed to the function or initializer.

simpleMin(17, 42) // T is inferred to be Int

simpleMin(3.14159, 2.71828) // T is inferred to be Double

Where Clauses

You can specify additional requirements on type parameters and their associated types by including a where clause after the generic parameter list. A where clause consists of the keyword where, followed by a comma-separated list of one or more requirements.

The requirements in a where clause specify that a type parameter inherits from a class or conforms to a protocol or protocol composition. Although the where clause provides syntactic sugar for expressing simple constraints on type parameters (for instance, T: Comparable is equivalent to T where T: Comparable and so on), you can use it to provide more complex constraints on type parameters and their associated types. For instance, you can express the constraints that a generic type T inherits from a class C and conforms to a

protocol P as <T where T: C, T: P>.

As mentioned above, you can constrain the associated types of type parameters to conform to protocols. For example, the generic parameter clause <T: Generator where T.Element: Equatable> specifies that T conforms to the Generator protocol and the associated type of T,

T.Element, conforms to the Equatable protocol (T has the associated type Element because

Generator declares Element and T conforms to Generator).

You can also specify the requirement that two types be identical, using the

For example, the generic parameter clause <T: Generator, U: Generator where T.Element ==

U.Element> expresses the constraints that T and U conform to the Generator protocol and that their associated types must be identical.

Any type argument substituted for a type parameter must meet all the constraints and requirements placed on the type parameter.

You can overload a generic function or initializer by providing different constraints, requirements, or both on the type parameters in the generic parameter clause. When you call an overloaded generic function or initializer, the compiler uses these constraints to resolve which overloaded function or initializer to invoke.

You can subclass a generic class, but the subclass must also be a generic class.

G R A M M A R O F A G E N E R I C P A R A M E T E R C L A U S E

​generic-parameter-clause → < generic-parameter-list requirement-clause opt > ​generic-parameter-list → generic-parameter generic-parameter , generic-parameter-

list

​generic-parameter → type-name ​generic-parameter → type-name : type-identifier

​generic-parameter → type-name : protocol-composition-type

​requirement-clause → where requirement-list

​requirement-list → requirement requirement , requirement-list ​requirement → conformance-requirement same-type-requirement

​conformance-requirement → type-identifier : type-identifier ​conformance-requirement → type-identifier : protocol-composition-type ​same-type-requirement → type-identifier == type-identifier

Generic Argument Clause

A generic argument clause specifies the type arguments of a generic type. A generic argument clause is enclosed in angle brackets (<>) and has the following form:

< generic argument list >

The generic argument list is a comma-separated list of type arguments. A type argument is the name of an actual concrete type that replaces a corresponding type parameter in the generic parameter clause of a generic type. The result is a specialized version of that generic type. As an example, the Swift standard library defines a generic dictionary type as:

struct Dictionary<KeyType: Hashable, ValueType>: Collection, DictionaryLiteralConvertible {

/* ... */

}

The specialized version of the generic Dictionary type, Dictionary<String, Int> is formed by replacing the generic parameters KeyType: Hashable and ValueType with the concrete type arguments String and Int. Each type argument must satisfy all the constraints of the generic parameter it replaces, including any additional requirements specified in a where clause. In the example above, the KeyType type parameter is constrained to conform to the Hashable protocol and therefore String must also conform to the Hashable protocol.

You can also replace a type parameter with a type argument that is itself a specialized version of a generic type (provided it satisfies the appropriate constraints and requirements). For example, you can replace the type parameter T in Array<T> with a specialized version of an array, Array<Int>, to form an array whose elements are themselves arrays of integers.

let arrayOfArrays: Array<Array<Int>> = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

As mentioned in Generic Parameter Clause, you don’t use a generic argument clause to specify the type arguments of a generic function or initializer.

G R A M M A R O F A G E N E R I C A R G U M E N T C L A U S E

​generic-argument-clause → < generic-argument-list >

​generic-argument-list → generic-argument generic-argument , generic-argument-list ​generic-argument → type

G R A M M A R O F A FA L LT H R O U G H S T A T E M E N T

​fallthrough-statement → fallthrough

G R A M M A R O F A R E T U R N S T A T E M E N T

​return-statement → return expressionopt

Generic Parameters and Arguments

G R A M M A R O F A G E N E R I C P A R A M E T E R C L A U S E

​generic-parameter-clause → < generic-parameter-list requirement-clause opt > ​generic-parameter-list → generic-parameter generic-parameter , generic-parameter-

list

​generic-parameter → type-name ​generic-parameter → type-name : type-identifier

​generic-parameter → type-name : protocol-composition-type

​requirement-clause → where requirement-list

​requirement-list → requirement requirement , requirement-list ​requirement → conformance-requirement same-type-requirement

​conformance-requirement → type-identifier : type-identifier ​conformance-requirement → type-identifier : protocol-composition-type ​same-type-requirement → type-identifier == type-identifier

G R A M M A R O F A G E N E R I C A R G U M E N T C L A U S E

​generic-argument-clause → < generic-argument-list >

​generic-argument-list → generic-argument generic-argument , generic-argument-list ​generic-argument → type

Declarations

G R A M M A R O F A D E C L A R A T I O N

​declaration → import-declaration ​declaration → constant-declaration ​declaration → variable-declaration ​declaration → typealias-declaration

​declaration → function-declaration ​declaration → enum-declaration ​declaration → struct-declaration ​declaration → class-declaration ​declaration → protocol-declaration ​declaration → initializer-declaration ​declaration → deinitializer-declaration ​declaration → extension-declaration ​declaration → subscript-declaration ​declaration → operator-declaration ​declarations → declaration declarations opt

​declaration-specifiers → declaration-specifier declaration-specifiers opt

​declaration-specifier → class mutating nonmutating override static unowned

unowned(safe) unowned(unsafe) weak

G R A M M A R O F A T O P - L E V E L D E C L A R A T I O N

​top-level-declaration → statements opt

G R A M M A R O F A C O D E B L O C K

​code-block → { statements opt }

G R A M M A R O F A N I M P O R T D E C L A R A T I O N

​import-declaration → attributes opt import import-kindopt import-path

​import-kind → typealias struct class enum protocol var func

​import-path → import-path-identifier import-path-identifier . import-path ​import-path-identifier → identifier operator

G R A M M A R O F A C O N S T A N T D E C L A R A T I O N

​constant-declaration → attributes opt declaration-specifiers opt let pattern-initializer-list

​pattern-initializer-list → pattern-initializer pattern-initializer , pattern-initializer-list ​pattern-initializer → pattern initializeropt

​initializer → = expression

G R A M M A R O F A V A R I A B L E D E C L A R A T I O N

​variable-declaration → variable-declaration-head pattern-initializer-list ​variable-declaration → variable-declaration-head variable-name type-

annotation code-block

​variable-declaration → variable-declaration-head variable-name typeannotation getter-setter-block

​variable-declaration → variable-declaration-head variable-name typeannotation getter-setter-keyword-block

​variable-declaration → variable-declaration-head variable-name typeannotation initializeropt willSet-didSet-block

​variable-declaration-head → attributes opt declaration-specifiers opt var ​variable-name → identifier

​getter-setter-block → { getter-clause setter-clause opt } ​getter-setter-block → { setter-clause getter-clause } ​getter-clause → attributes opt get code-block

​setter-clause → attributes opt set setter-name opt code-block ​setter-name → ( identifier )

​getter-setter-keyword-block → { getter-keyword-clause setter-keyword-clause opt } ​getter-setter-keyword-block → { setter-keyword-clause getter-keyword-clause } ​getter-keyword-clause → attributes opt get

​setter-keyword-clause → attributes opt set

​willSet-didSet-block → { willSet-clause didSet-clause opt } ​willSet-didSet-block → { didSet-clause willSet-clause } ​willSet-clause → attributes opt willSet setter-name opt code-block ​didSet-clause → attributes opt didSet setter-name opt code-block

G R A M M A R O F A T Y P E A L I A S D E C L A R A T I O N

​typealias-declaration → typealias-head typealias-assignment ​typealias-head → typealias typealias-name

​typealias-name → identifier ​typealias-assignment → = type

G R A M M A R O F A F U N C T I O N D E C L A R A T I O N

​function-declaration → function-head function-name generic-parameter- clause opt function-signature function-body

​function-head → attributes opt declaration-specifiers opt func ​function-name → identifier operator

​function-signature → parameter-clauses function-result opt ​function-result → -> attributes opt type

​function-body → code-block

​parameter-clauses → parameter-clause parameter-clauses opt ​parameter-clause → ( ) ( parameter-list ... opt ) ​parameter-list → parameter parameter , parameter-list

​parameter → inout opt let opt # opt parameter-name local-parameter-name opt typeannotation default-argument-clause opt

​parameter → inout opt var # opt parameter-name local-parameter-name opt typeannotation default-argument-clause opt

​parameter → attributes opt type ​parameter-name → identifier _ ​local-parameter-name → identifier _ ​default-argument-clause → = expression

G R A M M A R O F A N E N U M E R A T I O N D E C L A R A T I O N

​enum-declaration → attributes opt union-style-enum attributes opt raw-value-style- enum

​union-style-enum → enum-name generic-parameter-clause opt { union-style-enum-

members opt }

​union-style-enum-members → union-style-enum-member union-style-enum- members opt

​union-style-enum-member → declaration union-style-enum-case-clause ​union-style-enum-case-clause → attributes opt case union-style-enum-case-list ​union-style-enum-case-list → union-style-enum-case union-style-enum-case , union-

style-enum-case-list

​union-style-enum-case → enum-case-name tuple-type opt ​enum-name → identifier

​enum-case-name → identifier

​raw-value-style-enum → enum-name generic-parameter-clause opt : typeidentifier { raw-value-style-enum-members opt }

​raw-value-style-enum-members → raw-value-style-enum-member raw-value-style- enum-members opt

​raw-value-style-enum-member → declaration raw-value-style-enum-case-clause ​raw-value-style-enum-case-clause → attributes opt case raw-value-style-enum-case-

list

​raw-value-style-enum-case-list → raw-value-style-enum-case raw-value-style-enum- case , raw-value-style-enum-case-list