Never

A type that has no values and can’t be constructed.

You’re reading documentation from an older version of Swift 6.1.

A symbol with the same signature as this one exists in the latest stable release of Swift 6.1.

@frozen enum Never

Mangled symbol

ss5NeverO What are these?

FNV24 hash: G4XX

Use Never as the return type of a function that doesn’t return normally — for example, because it runs forever or terminates the program.

// An infinite loop never returns. func forever() -> Never {  while true {  print("I will print forever.")  } } // Calling fatalError(_:file:line:) unconditionally stops the program. func crashAndBurn() -> Never {  fatalError("Something very, very bad happened") }

A function that returns Never is called a nonreturning function. Closures, methods, computed properties, and subscripts can also be nonreturning.

There’s no way to create an instance of Never; this characteristic makes it an uninhabited type. You can use an uninhabited type like Never to represent states in your program that are impossible to reach during execution. Swift’s type system uses this information — for example, to reason about control statements in cases that are known to be unreachable.

let favoriteNumber: Result<Int, Never> = .success(42) switch favoriteNumber { case .success(let value):  print("My favorite number is", value) }

In the code above, favoriteNumber has a failure type of Never, indicating that it always succeeds. The switch statement is therefore exhaustive, even though it doesn’t contain a .failure case, because that case could never be reached.