Declare a function that doesn't return in Scala

Question

Is it possible to declare a "no return" function in Scala? I.e.

def abort(): ! = {
    System.exit(1);
}

(The ! in this example is taken from Rust, and it means: entering this function is a one-way trip and we'll never return from it)

Answer 1

This is exactly what the Nothing type represents—a method or expression that will never return a value. It's the type of an expression that throws an exception, for example:

scala> :type throw new Exception()
Nothing

Scala also provides a special ??? operator with this type, which is commonly used to get code to typecheck during development.

scala> :type ???
Nothing

Nothing is a subtype of everything else, so you can use an expression of type Nothing anywhere any type is expected.

Answer 2

Use Nothing:

def loop: Nothing = loop

Expressions of this type cannot return normally, but can go into infinite loops or throw exceptions. However, you can't use Nothing in your example, since System.exit has a signature saying it returns Unit. Instead, you could try something like this to make the compiler happy:

def abort(): Nothing = {
  System.exit(1);
  ???  // Unreachable
}