Which one is better? 1. ```class SomeThing { fun throwAnException(): Nothing { ... } } // ......

# getting-started

[Jason5lee]

Which one is better?

1.

class SomeThing {
    fun throwAnException(): Nothing { ... }
}

// ...
val s: SomeThing = ...
if (...) 
    s.throwAnException()

2.

class SomeThing {
    fun anException(): Exception { ... }
}

// ...
val s: SomeThing = ...
if (...) 
    throw s.anException()

[Sam]

Trick question, the answer is actually

val s: SomeThing = ...
if (...) 
    return s.someFailureResult()

😁

[Sam]

But in seriousness I don't think the choice between your options 1 and 2 makes a whole lot of difference. Maybe an argument could be made for preferring 1 because it avoids the possibility of accidentally creating the exception without throwing it.

[Stephan Schroeder]

the thing that I don't like in 1. and 2. is that SomeThing builds the exception, but doesn't contain the logic that triggers throwing it 🤔
And if SomeThing is some kind of exception builder, why instanciate it before it's clear that an exception will be thrown. So that's why I assume SomeThing only contains some data, you want to add to the exception message.
So maybe 3.

// data classes come with a useful toString()-method
data class SomeData(...)

// ...
val d: SomeData = ...

// or check(condition){"generate msg given data: $d"}
require(...) {"something went wrong given data: $d"}

[Casey Brooks]

The stdlib uses precondition functions check(), checkNotNull(), error(), etc. which follow the semantics of your #1 example. Given that it returns Nothing, the compiler is smart enough to give you smart casting, control flow analysis, etc. in response to calling s.throwAnException(). You won’t get that with #2, and it leaves you vulnerable to someone calling s.anException() but not actually throwing it

[ephemient]

I think both are useful, and you'll see that in stdlib as well: kotlin.Result.getOrThrow(), `.exceptionOrNull()`; kotlinx.coroutines.Deferred.getCompleted(), .getCompletionExceptionOrNull()

[nkiesel]

when in doubt, I would go with the 2nd approach because esp. Java compilers are not smart enough to understand that throwAnException() never returns