I know Nothing in Kotlin

When the Greek philosopher Socrates professed that “I know only one thing — that I know nothing,” he wasn’t exactly professing ignorance. It was an ancient formulation of the Dunning-Kruger effect. He had discovered that the more he learned, the wider the expanse of human knowledge seemed and the less that it seemed he knew.

A fool, on the other hand, might believe that they’re “not smart, but genius… and a very stable genius at that,” and that they know “only the best words.”

In this sense, it’s a good thing to know nothing. It’s also true in Kotlin.

During a recent code pairing session, I discovered that in Kotlin, there are numerous developers who have written code in the language for many years. And yet, they still don’t know Nothing.

Our pairing session became a big fuss about Nothing. We were working on some Repository code in our shared Kotlin Multiplatform Mobile (KMM) module of our Meetup for Organizers app. In this code, there was a repeated block at the end of every repository call to handle network, DNS, and timeout errors. We decided it would make sense to refactor this block into a function as we used it so often.

The repeated block in question handled a catch block. It had to re-throw any CancellationExceptions to allow cooperative cancellation of coroutines. Then we had to wrap any other exception and re-throw it as a custom exception type so that we can easily tell iOS a complete list of all types we intend to throw in a @Throws declaration. This allows exception handling and avoids crashing when our functions are called by Swift code. In essence, this code block always threw an Exception.

The pairing session became quite funny, like the Laurel and Hardy “Who’s on First” comedy routine, but you could tell there was Nothing more frustrating, too. It went something like this:

“Why am I getting this red text at the call site?”

“The return type is incorrect because you always throw an exception. You need to return Nothing.”

“I already am returning nothing.”

“No, you’re returning Unit. All Kotlin functions return Unit if you don’t declare a type. You need to return Nothing.”

“I don’t understand. I’m returning nothing from the function.”

“Just trust me. Add it to the return type of the function: Upper case N- lowercase o…”

“I’m so confused.”

It turns out that my colleagues, who had been coding in Kotlin for many years, had never been exposed to Nothing.

Nothing is weird. Nothing is esoteric. But Nothing is quite so useful in these rare cases.

I know Nothing.

“Nothing has no instances. You can use Nothing to represent ‘a value that never exists’: for example, if a function has the return type of Nothing, it means that it never returns (always throws an exception).”

— Kotlin Documentation

The official definition only seems to raise further questions for newcomers to the language.

Why can’t I just return Unit?

Returning Unit would be lying to the compiler, and the compiler will miss opportunities to do the right thing. For example, the code in our repository wouldn’t compile because we promised to return a type that wasn’t Unit from each function. Returning Nothing allows our parent function to compile and return the correct type.

What benefits do I get from Nothing?

You do, indeed, get something for Nothing. For one, not using Nothing misses out on correctness benefits like highlighting code as dead and unreachable if we run any code after our function that returns Nothing. Your IDE knows quickly that it stops at Nothing to show the right warnings. If we wrote the same code in Java, there would be no warnings.

Generics are also an area that is good for Nothing. If you want to create an object inside a templated sealed class that doesn’t have any type, its type is Nothing. Any other solution won’t compile.

// credit to Allan Caine @ Kotlin Academy
sealed class LinkedList<out T>  {
    data class Node<T>(val payload: T, var next: LinkedList<T> = EmptyList) : LinkedList<T>()
    object EmptyList : LinkedList<Nothing>() 
}

Nothing doesn’t only apply to functions where you always throw. Nothing is valuable for early returns, such as replicating the Swift guard() function in Kotlin:

// credit to Zac Sweers
inline infix fun <T> T?.guard(block: () -> Nothing): T {
  return this ?: block()
}
fun sweetNothings(input: String?) {
  val value = input guard {
    show("input is null!")
    return
  }
}

Why doesn’t Kotlin just determine automatically that my function returns Nothing and handle this for me?

As to my third and final question, automatically determining that a function returns Nothing would cause a number of problems. Both the compiler and IDE checks would take longer to run as they need to test every branch. Automated type inference would produce some surprising types that could lead to later bugs.

Finally, we should tell the creators of Kotlin, “Thanks for Nothing!”

Originally published on Colin The Shots blog.