iOS Developer Library — Prerelease


The Swift Programming Language

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In Swift, there are four kinds of expressions: prefix expressions, binary expressions, primary expressions, and postfix expressions. Evaluating an expression returns a value, causes a side effect, or both.

Prefix and binary expressions let you apply operators to smaller expressions. Primary expressions are conceptually the simplest kind of expression, and they provide a way to access values. Postfix expressions, like prefix and binary expressions, let you build up more complex expressions using postfixes such as function calls and member access. Each kind of expression is described in detail in the sections below.

Grammar of an expression

expression try-operator­opt­prefix-expression­binary-expressions­opt­

expression-list expression­ expression­expression-list­

Prefix Expressions

Prefix expressions combine an optional prefix operator with an expression. Prefix operators take one argument, the expression that follows them.

For information about the behavior of these operators, see Basic Operators and Advanced Operators.

For information about the operators provided by the Swift standard library, see Swift Standard Library Operators Reference.

In addition to the standard library operators, you use & immediately before the name of a variable that’s being passed as an in-out argument to a function call expression. For more information and to see an example, see In-Out Parameters.

Grammar of a prefix expression

prefix-expression prefix-operator­opt­postfix-expression­

prefix-expression in-out-expression­

in-out-expression identifier­

Try Operator

A try expression consists of the try operator followed by an expression that can throw an error. It has the following form:

  • try expression

A forced-try expression consists of the try! operator followed by an expression that can throw an error. If the expression marked by a forced-try operator throws an error, a runtime error is produced. It has the following form:

  • try! expression

For more information and to see examples of how to use try, see Throwing Errors and Catching and Handling Errors.

Grammar of a try expression

try-operator try­ try­

Binary Expressions

Binary expressions combine an infix binary operator with the expression that it takes as its left-hand and right-hand arguments. It has the following form:

  • left-hand argument operator right-hand argument

For information about the behavior of these operators, see Basic Operators and Advanced Operators.

For information about the operators provided by the Swift standard library, see Swift Standard Library Operators Reference.

Grammar of a binary expression

binary-expression binary-operator­prefix-expression­

binary-expression assignment-operator­try-operator­opt­prefix-expression­

binary-expression conditional-operator­try-operator­opt­prefix-expression­

binary-expression type-casting-operator­

binary-expressions binary-expression­binary-expressions­opt­

Assignment Operator

The assignment operator sets a new value for a given expression. It has the following form:

  • expression = value

The value of the expression is set to the value obtained by evaluating the value. If the expression is a tuple, the value must be a tuple with the same number of elements. (Nested tuples are allowed.) Assignment is performed from each part of the value to the corresponding part of the expression. For example:

  1. (a, _, (b, c)) = ("test", 9.45, (12, 3))
  2. // a is "test", b is 12, c is 3, and 9.45 is ignored

The assignment operator does not return any value.

Grammar of an assignment operator


Ternary Conditional Operator

The ternary conditional operator evaluates to one of two given values based on the value of a condition. It has the following form:

  • condition ? expression used if true : expression used if false

If the condition evaluates to true, the conditional operator evaluates the first expression and returns its value. Otherwise, it evaluates the second expression and returns its value. The unused expression is not evaluated.

For an example that uses the ternary conditional operator, see Ternary Conditional Operator.

Grammar of a conditional operator

conditional-operator expression­

Type-Casting Operators

There are four type-casting operators: the is operator, the as operator, the as? operator, and the as! operator.

They have the following form:

  • expression is type
  • expression as type
  • expression as? type
  • expression as! type

The is operator checks at runtime whether the expression can be downcast to the specified type. It returns true if the expression can be downcast to the specified type; otherwise, it returns false.

The as operator performs a cast when it is known at compile time that the cast always succeeds, such as upcasting or bridging. Upcasting lets you use an expression as an instance of its type’s supertype, without using an intermediate variable. The following approaches are equivalent:

  1. func f(any: Any) { print("Function for Any") }
  2. func f(int: Int) { print("Function for Int") }
  3. let x = 10
  4. f(x)
  5. // prints "Function for Int"
  6. let y: Any = x
  7. f(y)
  8. // prints "Function for Any"
  9. f(x as Any)
  10. // prints "Function for Any"

Bridging lets you use an expression of a Swift standard library type such as String as its corresponding Foundation type such as NSString without needing to create a new instance. For more information on bridging, see Working with Cocoa Data Types in Using Swift with Cocoa and Objective-C.

The as? operator performs a conditional cast of the expression to the specified type. The as? operator returns an optional of the specified type. At runtime, if the cast succeeds, the value of expression is wrapped in an optional and returned; otherwise, the value returned is nil. If casting to the specified type is guaranteed to fail or is guaranteed to succeed, a compile-time error is raised.

The as! operator performs a forced cast of the expression to the specified type. The as! operator returns a value of the specified type, not an optional type. If the cast fails, a runtime error is raised. The behavior of x as! T is the same as the behavior of (x as? T)!.

For more information about type casting and to see examples that use the type-casting operators, see Type Casting.

Grammar of a type-casting operator

type-casting-operator is­type­

type-casting-operator as­type­

type-casting-operator as­type­

type-casting-operator as­type­

Primary Expressions

Primary expressions are the most basic kind of expression. They can be used as expressions on their own, and they can be combined with other tokens to make prefix expressions, binary expressions, and postfix expressions.

Grammar of a primary expression

primary-expression identifier­generic-argument-clause­opt­

primary-expression literal-expression­

primary-expression self-expression­

primary-expression superclass-expression­

primary-expression closure-expression­

primary-expression parenthesized-expression­

primary-expression implicit-member-expression­

primary-expression wildcard-expression­

Literal Expression

A literal expression consists of either an ordinary literal (such as a string or a number), an array or dictionary literal, or one of the following special literals:






The name of the file in which it appears.



The line number on which it appears.



The column number in which it begins.



The name of the declaration in which it appears.

Inside a function, the value of __FUNCTION__ is the name of that function, inside a method it is the name of that method, inside a property getter or setter it is the name of that property, inside special members like init or subscript it is the name of that keyword, and at the top level of a file it is the name of the current module.

When used as the default value of a function or method, the special literal’s value is determined when the default value expression is evaluated at the call site.

  1. func logFunctionName(string: String = __FUNCTION__) {
  2. print(string)
  3. }
  4. func myFunction() {
  5. logFunctionName() // Prints "myFunction()".
  6. }
  7. myFunction()
  8. namedArgs(1, withJay: 2)

An array literal is an ordered collection of values. It has the following form:

  • [value 1, value 2, ...]

The last expression in the array can be followed by an optional comma. The value of an array literal has type [T], where T is the type of the expressions inside it. If there are expressions of multiple types, T is their closest common supertype. Empty array literals are written using an empty pair of square brackets and can be used to create an empty array of a specified type.

  1. var emptyArray: [Double] = []

A dictionary literal is an unordered collection of key-value pairs. It has the following form:

  • [key 1: value 1, key 2: value 2, ...]

The last expression in the dictionary can be followed by an optional comma. The value of a dictionary literal has type [Key: Value], where Key is the type of its key expressions and Value is the type of its value expressions. If there are expressions of multiple types, Key and Value are the closest common supertype for their respective values. An empty dictionary literal is written as a colon inside a pair of brackets ([:]) to distinguish it from an empty array literal. You can use an empty dictionary literal to create an empty dictionary literal of specified key and value types.

  1. var emptyDictionary: [String: Double] = [:]

Grammar of a literal expression

literal-expression literal­

literal-expression array-literal­ dictionary-literal­

literal-expression __FILE__­ __LINE__­ __COLUMN__­ __FUNCTION__­

array-literal array-literal-items­opt­

array-literal-items array-literal-item­opt­ array-literal-item­array-literal-items­

array-literal-item expression­

dictionary-literal dictionary-literal-items­

dictionary-literal-items dictionary-literal-item­opt­ dictionary-literal-item­dictionary-literal-items­

dictionary-literal-item expression­expression­

Self Expression

The self expression is an explicit reference to the current type or instance of the type in which it occurs. It has the following forms:

  • self
  • self.member name
  • self[subscript index]
  • self(initializer arguments)
  • self.init(initializer arguments)

In an initializer, subscript, or instance method, self refers to the current instance of the type in which it occurs. In a type method, self refers to the current type in which it occurs.

The self expression is used to specify scope when accessing members, providing disambiguation when there is another variable of the same name in scope, such as a function parameter. For example:

  1. class SomeClass {
  2. var greeting: String
  3. init(greeting: String) {
  4. self.greeting = greeting
  5. }
  6. }

In a mutating method of a value type, you can assign a new instance of that value type to self. For example:

  1. struct Point {
  2. var x = 0.0, y = 0.0
  3. mutating func moveByX(deltaX: Double, y deltaY: Double) {
  4. self = Point(x: x + deltaX, y: y + deltaY)
  5. }
  6. }

Grammar of a self expression

self-expression self­

self-expression self­identifier­

self-expression self­expression­

self-expression self­init­

Superclass Expression

A superclass expression lets a class interact with its superclass. It has one of the following forms:

  • super.member name
  • super[subscript index]
  • super.init(initializer arguments)

The first form is used to access a member of the superclass. The second form is used to access the superclass’s subscript implementation. The third form is used to access an initializer of the superclass.

Subclasses can use a superclass expression in their implementation of members, subscripting, and initializers to make use of the implementation in their superclass.

Grammar of a superclass expression

superclass-method-expression super­identifier­

superclass-subscript-expression super­expression­

superclass-initializer-expression super­init­

Closure Expression

A closure expression creates a closure, also known as a lambda or an anonymous function in other programming languages. Like function declarations, closures contain statements which they execute, and they capture values from their enclosing scope. It has the following form:

  • { (parameters) -> return type in
  •     statements
  • }

The parameters have the same form as the parameters in a function declaration, as described in Function Declaration.

There are several special forms that allow closures to be written more concisely:

  • A closure can omit the types of its parameters, its return type, or both. If you omit the parameter names and both types, omit the in keyword before the statements. If the omitted types can’t be inferred, a compile-time error is raised.

  • A closure may omit names for its parameters. Its parameters are then implicitly named $ followed by their position: $0, $1, $2, and so on.

  • A closure that consists of only a single expression is understood to return the value of that expression. The contents of this expression are also considered when performing type inference on the surrounding expression.

The following closure expressions are equivalent:

  1. myFunction {
  2. (x: Int, y: Int) -> Int in
  3. return x + y
  4. }
  5. myFunction {
  6. (x, y) in
  7. return x + y
  8. }
  9. myFunction { return $0 + $1 }
  10. myFunction { $0 + $1 }

For information about passing a closure as an argument to a function, see Function Call Expression.

A closure expression can explicitly specify the values that it captures from the surrounding scope using a capture list. A capture list is written as a comma separated list surrounded by square brackets, before the list of parameters. If you use a capture list, you must also use the in keyword, even if you omit the parameter names, parameter types, and return type.

Each entry in the capture list can be marked as weak or unowned to capture a weak or unowned reference to the value.

  1. myFunction { print(self.title) } // strong capture
  2. myFunction { [weak self] in print(self!.title) } // weak capture
  3. myFunction { [unowned self] in print(self.title) } // unowned capture

You can also bind an arbitrary expression to a named value in the capture list. The expression is evaluated when the closure is formed, and captured with the specified strength. For example:

  1. // Weak capture of "self.parent" as "parent"
  2. myFunction { [weak parent = self.parent] in print(parent!.title) }

For more information and examples of closure expressions, see Closure Expressions.

Grammar of a closure expression

closure-expression closure-signature­opt­statements­

closure-signature parameter-clause­function-result­opt­in­

closure-signature identifier-list­function-result­opt­in­

closure-signature capture-list­parameter-clause­function-result­opt­in­

closure-signature capture-list­identifier-list­function-result­opt­in­

closure-signature capture-list­in­

capture-list capture-specifier­expression­

capture-specifier weak­ unowned­ unowned(safe)­ unowned(unsafe)­

Implicit Member Expression

An implicit member expression is an abbreviated way to access a member of a type, such as an enumeration case or a type method, in a context where type inference can determine the implied type. It has the following form:

  • .member name

For example:

  1. var x = MyEnumeration.SomeValue
  2. x = .AnotherValue

Grammar of a implicit member expression

implicit-member-expression identifier­

Parenthesized Expression

A parenthesized expression consists of a comma-separated list of expressions surrounded by parentheses. Each expression can have an optional identifier before it, separated by a colon (:). It has the following form:

  • (identifier 1: expression 1, identifier 2: expression 2, ...)

Use parenthesized expressions to create tuples and to pass arguments to a function call. If there is only one value inside the parenthesized expression, the type of the parenthesized expression is the type of that value. For example, the type of the parenthesized expression (1) is Int, not (Int).

Grammar of a parenthesized expression

parenthesized-expression expression-element-list­opt­

expression-element-list expression-element­ expression-element­expression-element-list­

expression-element expression­ identifier­expression­

Wildcard Expression

A wildcard expression is used to explicitly ignore a value during an assignment. For example, in the following assignment 10 is assigned to x and 20 is ignored:

  1. (x, _) = (10, 20)
  2. // x is 10, and 20 is ignored

Grammar of a wildcard expression


Postfix Expressions

Postfix expressions are formed by applying a postfix operator or other postfix syntax to an expression. Syntactically, every primary expression is also a postfix expression.

For information about the behavior of these operators, see Basic Operators and Advanced Operators.

For information about the operators provided by the Swift standard library, see Swift Standard Library Operators Reference.

Grammar of a postfix expression

postfix-expression primary-expression­

postfix-expression postfix-expression­postfix-operator­

postfix-expression function-call-expression­

postfix-expression initializer-expression­

postfix-expression explicit-member-expression­

postfix-expression postfix-self-expression­

postfix-expression dynamic-type-expression­

postfix-expression subscript-expression­

postfix-expression forced-value-expression­

postfix-expression optional-chaining-expression­

Function Call Expression

A function call expression consists of a function name followed by a comma-separated list of the function’s arguments in parentheses. Function call expressions have the following form:

  • function name(argument value 1, argument value 2)

The function name can be any expression whose value is of a function type.

If the function definition includes names for its parameters, the function call must include names before its argument values separated by a colon (:). This kind of function call expression has the following form:

  • function name(argument name 1: argument value 1, argument name 2: argument value 2)

A function call expression can include a trailing closure in the form of a closure expression immediately after the closing parenthesis. The trailing closure is understood as an argument to the function, added after the last parenthesized argument. The following function calls are equivalent:

  1. // someFunction takes an integer and a closure as its arguments
  2. someFunction(x, f: {$0 == 13})
  3. someFunction(x) {$0 == 13}

If the trailing closure is the function’s only argument, the parentheses can be omitted.

  1. // someFunction takes a closure as its only argument
  2. myData.someMethod() {$0 == 13}
  3. myData.someMethod {$0 == 13}

Grammar of a function call expression

function-call-expression postfix-expression­parenthesized-expression­

function-call-expression postfix-expression­parenthesized-expression­opt­trailing-closure­

trailing-closure closure-expression­

Initializer Expression

An initializer expression provides access to a type’s initializer. It has the following form:

  • expression.init(initializer arguments)

You use the initializer expression in a function call expression to initialize a new instance of a type. Unlike functions, an initializer can’t be used as a value. For example:

  1. var x = SomeClass.someClassFunction // ok
  2. var y = SomeClass.init // error

You also use an initializer expression to delegate to the initializer of a superclass.

  1. class SomeSubClass: SomeSuperClass {
  2. override init() {
  3. // subclass initialization goes here
  4. super.init()
  5. }
  6. }

Grammar of an initializer expression

initializer-expression postfix-expression­init­

Explicit Member Expression

A explicit member expression allows access to the members of a named type, a tuple, or a module. It consists of a period (.) between the item and the identifier of its member.

  • expression.member name

The members of a named type are named as part of the type’s declaration or extension. For example:

  1. class SomeClass {
  2. var someProperty = 42
  3. }
  4. let c = SomeClass()
  5. let y = c.someProperty // Member access

The members of a tuple are implicitly named using integers in the order they appear, starting from zero. For example:

  1. var t = (10, 20, 30)
  2. t.0 = t.1
  3. // Now t is (20, 20, 30)

The members of a module access the top-level declarations of that module.

Grammar of an explicit member expression

explicit-member-expression postfix-expression­decimal-digits­

explicit-member-expression postfix-expression­identifier­generic-argument-clause­opt­

Postfix Self Expression

A postfix self expression consists of an expression or the name of a type, immediately followed by .self. It has the following forms:

  • expression.self
  • type.self

The first form evaluates to the value of the expression. For example, x.self evaluates to x.

The second form evaluates to the value of the type. Use this form to access a type as a value. For example, because SomeClass.self evaluates to the SomeClass type itself, you can pass it to a function or method that accepts a type-level argument.

Grammar of a self expression

postfix-self-expression postfix-expression­self­

Dynamic Type Expression

A dynamicType expression consists of an expression, immediately followed by .dynamicType. It has the following form:

  • expression.dynamicType

The expression can’t be the name of a type. The entire dynamicType expression evaluates to the value of the runtime type of the expression, as the following example shows:

  1. class SomeBaseClass {
  2. class func printClassName() {
  3. print("SomeBaseClass")
  4. }
  5. }
  6. class SomeSubClass: SomeBaseClass {
  7. override class func printClassName() {
  8. print("SomeSubClass")
  9. }
  10. }
  11. let someInstance: SomeBaseClass = SomeSubClass()
  12. // someInstance has a static type of SomeBaseClass at compile time, and
  13. // it has a dynamc type of SomeSubClass at runtime
  14. someInstance.dynamicType.printClassName()
  15. // prints "SomeSubClass"

Grammar of a dynamic type expression

dynamic-type-expression postfix-expression­dynamicType­

Subscript Expression

A subscript expression provides subscript access using the getter and setter of the corresponding subscript declaration. It has the following form:

  • expression[index expressions]

To evaluate the value of a subscript expression, the subscript getter for the expression’s type is called with the index expressions passed as the subscript parameters. To set its value, the subscript setter is called in the same way.

For information about subscript declarations, see Protocol Subscript Declaration.

Grammar of a subscript expression

subscript-expression postfix-expression­expression-list­

Forced-Value Expression

A forced-value expression unwraps an optional value that you are certain is not nil. It has the following form:

  • expression!

If the value of the expression is not nil, the optional value is unwrapped and returned with the corresponding nonoptional type. Otherwise, a runtime error is raised.

The unwrapped value of a forced-value expression can be modified, either by mutating the value itself, or by assigning to one of the value’s members. For example:

  1. var x: Int? = 0
  2. x!++
  3. // x is now 1
  4. var someDictionary = ["a": [1, 2, 3], "b": [10, 20]]
  5. someDictionary["a"]![0] = 100
  6. // someDictionary is now [b: [10, 20], a: [100, 2, 3]]

Grammar of a forced-value expression

forced-value-expression postfix-expression­

Optional-Chaining Expression

An optional-chaining expression provides a simplified syntax for using optional values in postfix expressions. It has the following form:

  • expression?

Optional-chaining expressions must appear within a postfix expression, and they cause the postfix expression to be evaluated in a special way. If the optional-chaining expression is nil, all of the other operations in the postfix expression are ignored and the entire postfix expression evaluates to nil. If the optional-chaining expression is not nil, the value of the optional-chaining expression is unwrapped and used to evaluate the rest of the postfix expression. In either case, the value of the postfix expression is still of an optional type.

If a postfix expression that contains an optional-chaining expression is nested inside other postfix expressions, only the outermost expression returns an optional type. In the example below, when c is not nil, its value is unwrapped and used to evaluate .property, the value of which is used to evaluate .performAction(). The entire expression c?.property.performAction() has a value of an optional type.

  1. var c: SomeClass?
  2. var result: Bool? = c?.property.performAction()

The following example shows the behavior of the example above without using optional chaining.

  1. var result: Bool? = nil
  2. if let unwrappedC = c {
  3. result =
  4. }

The unwrapped value of an optional-chaining expression can be modified, either by mutating the value itself, or by assigning to one of the value’s members. If the value of the optional-chaining expression is nil, the expression on the right hand side of the assignment operator is not evaluated. For example:

  1. func someFunctionWithSideEffects() -> Int {
  2. return 42 // No actual side effects.
  3. }
  4. var someDictionary = ["a": [1, 2, 3], "b": [10, 20]]
  5. someDictionary["not here"]?[0] = someFunctionWithSideEffects()
  6. // someFunctionWithSideEffects is not evaluated
  7. // someDictionary is still [b: [10, 20], a: [1, 2, 3]]
  8. someDictionary["a"]?[0] = someFunctionWithSideEffects()
  9. // someFunctionWithSideEffects is evaluated and returns 42
  10. // someDictionary is now [b: [10, 20], a: [42, 2, 3]]

Grammar of an optional-chaining expression

optional-chaining-expression postfix-expression­