Why vararg Nothing is forbidden

Question

cketti · Answer

If the goal is to force callers to use named parameters you should be able to accomplish that by creating your own class that you won t allow anyone to instantiate

Jonathan · Answer

Because it is not possible to get an instance of Nothing using it in the argument of a function mean you don t want users to be able to call the function Or in this case you you don t want users to pass any `ign` argument so why declaring the argument in the first place

marcinmoskala · Answer

As < cketti> guessed to force users to use named parameters

marcinmoskala · Answer

Using `Nothing` would be easier and more clear confused

cketti · Answer

the fact that many people didn t pick up what you re trying to accomplish probably means it s not very clear slightly smiling face

cketti · Answer

you could name the type `ForceCallersToUseNamedParameters` that ll make it somewhat clear

marcinmoskala · Answer

stuck out tongue

marcinmoskala · Answer

I am just experimenting with this idea I took it from Effective Python where author suggested that pattern to force named parameters usage Though in Python all you need it ` ` Also I think that it is not in the philosophy of Kotlin In Kotlin function user should decide if he prefers more or less explicitly Sometimes it would be redundant if user would extract variable with the same say anyway Though good rationale is that this way function designer can always change parameters order On the other hand for this we now have inline classes that in most cases should help us from passing value to a wrong parameter It is not so simple and I am just experimenting now smile

marcinmoskala · Answer

Formally in JVM `Nothing` parameter is translated to `Void` so it should work with `vararg` No one really answered the question why it is forbidden

cketti · Answer

it doesn t make much sense to allow creating a function that you can t call you found an edge case where it could be possible to call the method supporting that doesn t seem worthwhile from a language standpoint

Allan Wang · Answer

You could also just make an object with a long name like `ForceNamedParameters` as mentioned above and take that as input It better describes the situation

Allan Wang · Answer

``` object ForceNamedParameters fun aaa forceNamedParameters ForceNamedParameters = ForceNamedParameters a Int b Int ```

Jonathan · Answer

I d declare a class with private constructor `class ForceNamedParameters private constructor ` But even with a properly named parameter declaring an argument which is not an actual argument obfuscate the code IMO I think it s up to the user of the function to decide weather it is worthwhile or not to use named parameters I don t understand the needs to enforce it

Jonathan · Answer

In most all cases types of arguments should be enough for safe usage of the function Simply avoid the primitive obsession smell

louiscad · Answer

Actually if `vararg something Nothing` was allowed you could call it because you could call it with zero parameters It s just an `Array lt Nothing gt ` under the hood which can be empty