AnyStruct, AnyResource, Optionals, and Never

AnyStruct is the top type of all non-resource types (i.e., all non-resource types are a subtype of it).

AnyResource is the top type of all resource types.

_37

// Declare a variable that has the type `AnyStruct`.

_37

// Any non-resource typed value can be assigned to it, for example an integer,

_37

// but not resource-typed values.

_37

//

_37

var someStruct: AnyStruct = 1

_37

_37

// Assign a value with a different non-resource type, `Bool`.

_37

someStruct = true

_37

_37

// Declare a structure named `TestStruct`, create an instance of it,

_37

// and assign it to the `AnyStruct`-typed variable

_37

//

_37

struct TestStruct {}

_37

_37

let testStruct = TestStruct()

_37

_37

someStruct = testStruct

_37

_37

// Declare a resource named `TestResource`

_37

_37

resource TestResource {}

_37

_37

// Declare a variable that has the type `AnyResource`.

_37

// Any resource-typed value can be assigned to it,

_37

// but not non-resource typed values.

_37

//

_37

var someResource: @AnyResource <- create TestResource()

_37

_37

// Invalid: Resource-typed values can not be assigned

_37

// to `AnyStruct`-typed variables

_37

//

_37

someStruct <- create TestResource()

_37

_37

// Invalid: Non-resource typed values can not be assigned

_37

// to `AnyResource`-typed variables

_37

//

_37

someResource = 1

However, using AnyStruct and AnyResource does not allow you to opt out of type checking. It is invalid to access fields and call functions on these types, as they have no fields and functions.

_10

// Declare a variable that has the type `AnyStruct`.

_10

// The initial value is an integer,

_10

// but the variable still has the explicit type `AnyStruct`.

_10

//

_10

let a: AnyStruct = 1

_10

_10

// Invalid: Operator cannot be used for an `AnyStruct` value (`a`, left-hand side)

_10

// and an `Int` value (`2`, right-hand side).

_10

//

_10

a + 2

AnyStruct and AnyResource may be used like other types. For example, they may be the element type of arrays or be the element type of an optional type.

_13

// Declare a variable that has the type `[AnyStruct]`,

_13

// i.e. an array of elements of any non-resource type.

_13

//

_13

let anyValues: [AnyStruct] = [1, "2", true]

_13

_13

// Declare a variable that has the type `AnyStruct?`,

_13

// i.e. an optional type of any non-resource type.

_13

//

_13

var maybeSomething: AnyStruct? = 42

_13

_13

maybeSomething = "twenty-four"

_13

_13

maybeSomething = nil

AnyStruct is also the super-type of all non-resource optional types, and AnyResource is the super-type of all resource optional types.

_10

let maybeInt: Int? = 1

_10

let anything: AnyStruct = maybeInt

Conditional downcasting allows coercing a value that has the type AnyStruct or AnyResource back to its original type.

Optionals

Optionals are values which can represent the absence of a value. Optionals have two cases: either there is a value or there is nothing.

An optional type is declared using the ? suffix for another type. For example, Int is a non-optional integer and Int? is an optional integer (i.e., either nothing or an integer).

The value representing nothing is nil.

_17

// Declare a constant which has an optional integer type,

_17

// with nil as its initial value.

_17

//

_17

let a: Int? = nil

_17

_17

// Declare a constant which has an optional integer type,

_17

// with 42 as its initial value.

_17

//

_17

let b: Int? = 42

_17

_17

// Invalid: `b` has type `Int?`, which does not support arithmetic.

_17

b + 23

_17

_17

// Invalid: Declare a constant with a non-optional integer type `Int`,

_17

// but the initial value is `nil`, which in this context has type `Int?`.

_17

//

_17

let x: Int = nil

Optionals can be created for any value, not just for literals.

_15

// Declare a constant which has a non-optional integer type,

_15

// with 1 as its initial value.

_15

//

_15

let x = 1

_15

_15

// Declare a constant which has an optional integer type.

_15

// An optional with the value of `x` is created.

_15

//

_15

let y: Int? = x

_15

_15

// Declare a variable which has an optional any type, i.e. the variable

_15

// may be `nil`, or any other value.

_15

// An optional with the value of `x` is created.

_15

//

_15

var z: AnyStruct? = x

A non-optional type is a subtype of its optional type.

_10

var a: Int? = nil

_10

let b = 2

_10

a = b

_10

_10

// `a` is `2`

Optional types may be contained in other types (e.g., arrays or even optionals.)

_10

// Declare a constant which has an array type of optional integers.

_10

let xs: [Int?] = [1, nil, 2, nil]

_10

_10

// Declare a constant which has a double optional type.

_10

//

_10

let doubleOptional: Int?? = nil

See the optional operators section for information on how to work with optionals.

Never

Never is the bottom type (i.e., it is a subtype of all types). There is no value that has type Never.

Never can be used as the return type for functions that never return normally. For example, it is the return type of the function panic.

_17

// Declare a function named `crashAndBurn` which will never return,

_17

// because it calls the function named `panic`, which never returns.

_17

//

_17

fun crashAndBurn(): Never {

_17

panic("An unrecoverable error occurred")

_17

}

_17

_17

// Invalid: Declare a constant with a `Never` type, but the initial value is an integer.

_17

//

_17

let x: Never = 1

_17

_17

// Invalid: Declare a function which returns an invalid return value `nil`,

_17

// which is not a value of type `Never`.

_17

//

_17

fun returnNever(): Never {

_17

return nil

_17

}