Hi, does anyone know why the `!!` is necessary in ...

# getting-started

[Matias Reparaz]

Hi, does anyone know why the !! is necessary in the second branch of the when expression in this code? Isn’t there enough information for a smart cast to a non-nullable type?

fun checkSomething(old: Something?, new: Something?): Something = when {
    old == null && new == null -> throw IllegalArgumentException("Both old and new arguments are null")
    old == null -> new!!
    new == null -> old
    else -> old
}

[Vampire]

Theoretically, for a human brain, probably.
But the Kotlin compiler can probably not cover all possible cases a human brain can handle. 😄
Maybe somewhen in the future. 🤷‍♂️

[ephemient]

for this, if you change the order, then smart-casting will work

when {
    old != null -> old
    new != null -> new
    else -> throw IllegalArgumentException("Both old and new arguments are null")
}

or of course simply

requireNotNull(old ?: new)

perhaps your real use case is more complex?

[Matias Reparaz]

Yeah, this is just a simple example. I can rearrange the conditions like this

fun checkSomething(old: Something?, new: Something?): Something = when {
    old != null && new != null -> doSomething(old, new) // both smart casted as not nullable
    new != null -> doAnotherThing(new) // smart casted as not nullable
    old != null -> doAnotherThing(old) // smart casted as not nullable
    else -> throw IllegalArgumentException("Both old and new arguments are null")
}

but I’m surprised that the original code doesn’t work

[Sam]

I wonder if it's something that'll be improved by https://youtrack.jetbrains.com/issue/KT-63696

[Matias Reparaz]

Yeah, while looking through the related issues, I found this one, which I think is more specific to what I was referring to 👍

[Matias Reparaz]

as I mentioned, it’s not a major issue since I can reorder the conditions to enforce the smart cast, but it would be a nice feature to have

[Chrimaeon]

I think its more about the order logical expressions are evaluated.
if the first term evaluates to false the second one is not going to be evaluated. so in your initial example with the && if old == null the compiler cannot say anything about the value of new.

example with a basic if expression. https://pl.kotl.in/5M83m7RwW

[Matias Reparaz]

ok, that’s a very good point

[Vampire]

Why can it not?
The compiler could know that if the first path is taken both are null and that if the second path is taken only old is null as otherwise the first path would have been taken.
But I still think it is just not aware as that needs interpretation skills that are trivial for human but not for the compiler unless explicitly told. 🤷‍♂️

[Vampire]

For example like this: https://github.com/typesafegithub/github-actions-typing/commit/9e0cc6c0c536f4be2fb61047b3ac0760e9010656

[Vampire]

I did not include the start script creator task as that project is started using Docker, but it would just be registering and wiring one additional task