了解scala.Nothing类型

Question

According to the scala specification, scala.Nothing type - buttom for all types. Type "Nothing" exists but instance of Nothing does not exists.

How it works:

def ??? : Nothing = throw new NoImplementedError
def sys.error(message: String): Nothing = throw new RuntimeException()
def sys.exit(status: Int): Nothing = {...}

But in fact, all of mentioned methods return exception. Excepted def sys.exit Could you please clarify more about type Nothing. Any examples, explanations.

Thanks!

Answer 1

def ??? : Nothing = throw new NoImplementedError

does not return an exception, it throws an exception which is not the same thing. Exceptions are a control-flow mechanism which causes control to immediately jump to the nearest installed handler within the call stack. This means that in

val x = ???

x will never be assigned a value, so x can have any type at all. This type is Nothing in the scala type system which is the subtype of all types.

Non-termination also has type nothing since it also never returns a value eg

def loop(): Nothing = loop()

val x: Int = loop()

is therefore allowed since x will never be assigned.

Answer 2

In Scala absolutely everything needs to have a return type.

println for example have the return type Unit because it's a unit of calculations that returns no value.

If everything needs to have a return type then, how would you implement a function like ??? , so that it can be used by any function , no matter what type it needs to return?

Well that is what Nothing is for, it's a subtype of every other types (as Any is the top type of every other type), so a function that returns Nothing can be used anywhere and the type system will always stay consistent.

As you already noticed functions returning Nothing never return a value. That is because there is no value instenciable for Nothing .
You can see it as a way to make the compiler "happy" for functions that only throws or stop the app or loop forever.

---

In case you wonder why we don't use Unit instead of Nothing , here is an example:

def notYetImplemented: Unit = throw new Exception() 
def myFunc: String = notYetImplemented

This will not compile because Unit is not a String or a subtype of String . So you will need to do:

def myFunc: String = {
  notYetImplemented
  null
}

Which is not very convenient and for us to write a value to return even taught we will never reach this code.

Cheers