Conditional Downcasting Operator

🚧 Status: The conditional downcasting operator as? is implemented, but it only supports values that have the type AnyStruct and AnyResource.

The conditional downcasting operator as? can be used to type cast a value to a type. The operator returns an optional. If the value has a type that is a subtype of the given type that should be casted to, the operator returns the value as the given type, otherwise the result is nil.

The cast and check is performed at run-time, i.e. when the program is executed, not statically, i.e. when the program is checked.

// Declare a constant named `something` which has type `AnyStruct`,
// with an initial value which has type `Int`.
//
let something: AnyStruct = 1

// Conditionally downcast the value of `something` to `Int`.
// The cast succeeds, because the value has type `Int`.
//
let number = something as? Int
// `number` is `1` and has type `Int?`

// Conditionally downcast the value of `something` to `Bool`.
// The cast fails, because the value has type `Int`,
// and `Bool` is not a subtype of `Int`.
//
let boolean = something as? Bool
// `boolean` is `nil` and has type `Bool?`

Downcasting works for nested types (e.g. arrays), interfaces (if a resource interface not to a concrete resource), and optionals.

// Declare a constant named `values` which has type `[AnyStruct]`,
// i.e. an array of arbitrarily typed values.
//
let values: [AnyStruct] = [1, true]

let first = values[0] as? Int
// `first` is `1` and has type `Int?`

let second = values[1] as? Bool
// `second` is `true` and has type `Bool?`