just curious—anyone have a use case for using `Not...

# announcements

[Matthew Pope]

just curious—anyone have a use case for using Nothing? in your code anywhere?

[Zach Klippenstein (he/him) [MOD]]

A few times. Usually comes up as a specialization of a generic type.

e.g.:

sealed class Stuff<out T> {
  data class Something<T>(val it: T) : Stuff<T>() {
    override fun getOrNull(): T? = it
  }

  object None : Stuff<Nothing>() {
    override fun getOrNull(): Nothing? = null
  }

  abstract fun getOrNull(): T?
}

[nanodeath]

slightly OT: I've discovered that val foo = null does in fact compile, with type Nothing? :)

[nanodeath]

(not that it's useful)

[Matthew Pope]

@nanodeath I asked out of curiosity, and you’ve told me something interesting. That’s on topic in my books.

[Zach Klippenstein (he/him) [MOD]]

I think it’s interesting that while Unit happens to be defined as a type with a single value (like any object), Nothing? is a type that can only have one value.

[Matthew Pope]

@Zach Klippenstein (he/him) [MOD] thanks. That actually makes a lot of sense.

[streetsofboston]

Nothing is a type without any values (empty set).

T? is a type whose values are in T or is null.

Nothing? has therefore one value (any value in Nothing - which don't exist - or null), and that value is null.

Conversely, the value null has the type Nothing?, and Nothing? is a sub-type of all other types T?. That is why you can assign the value null to every nullable type (T?).
😀

[Zach Klippenstein (he/him) [MOD]]

Types are fun! 😂

[Matthew Pope]

I have it—a use case for Nothing? that is not forced by an interface or some weird generics, but is still totally contrived. 😏 Consider this code snippet:

when (it) {
    null -> println("It's null")
    is Int -> println("It's an int")
    is Long -> println("It's a long")
}

Maybe you think that is ugly because the first condition is not a type check like the rest of them… you can substitute is Nothing? for the null equality check.

[Zach Klippenstein (he/him) [MOD]]

Interesting as code golf, but I'd find is Nothing? there a lot more confusing than a simple null. The null makes it really clear it's a null check, using Nothing forces you to think much harder.

[Fabio]

I think it's used with methods that never return, like

while(true) {
// do work
}

Arduino coders may love this trick 😉

[Zach Klippenstein (he/him) [MOD]]

Expressions that never complete have the type Nothing (not nullable), because they can't ever have a value. When the compiler encounters an expression of type Nothing, it knows nothing after it will ever be executed, which is how it can warn about dead code after a return, for example. If something has the type Nothing?, it can still have a value, and code after it might still execute.

[araqnid]

so return and throw are basically expressions of type Nothing (which is why you can put TODO() anywhere, and code after it is dead)

[Fabio]

I think return and throw are Unit, but not sure here.
Basically, there's a single object Unit but there's no object that can be of type Nothing

[streetsofboston]

No, Steve is correct. The return and throw are keywords forming an expression that returns Nothing (not even a Unit) and no code can follow it. Eg return return return return return 5 is an expression that compiles fine...😀

[araqnid]

.. and that’s why val thing = findThing(id) ?: throw NotFoundException() works

[dmitriy.novozhilov]

There is one usecase where Nothing? can be usefull. Assume you have some parametrized visitor

abstract class Visitor<R, D> {
    abstract fun visitElement(
        element: Element,
        data: D
    ): R
    ...
}

and you want to have visitor which doesn't use any data. In this case you can parametrize D with Nothing?

class MyVisitor : Visitor<String, Nothing?>() {
    override fun visitElement(
        element: Element,
        data: Nothing?
    ): String {
        ...
    }
    ...
}

fun test(element: Element) {
    val visitor = MyVisitor()
    val result = element.accept(visitor, null)
}

Also Unit can be used in this case instead of Nothing?, but it depends of your habbits and codestyle

[Zach Klippenstein (he/him) [MOD]]

I sometimes think of it like: Unit means the author/API explicitly intends there to be only a single possible value. Nothing? means that there are probably some combination of type constraints that mean there won't be a value here, but with other type constraints there probably could be a value.