In my experience `as ` or `as` doesn t seem to work when usi

Question

In my experience `as ` or `as` doesn t seem to work when using them with generic `T KClass lt out Any gt ` For example `someClass as T` will return `someClass` even when `T` is not `someClass` s ancestor if anyone could put an eye on the code in the comment to this that would be great Currently in trying to achieve `instanceof` like behavior my method returns `true` for `Nothing class instanceof Number class` and I wish I was joking robot face I ve been trying to figure this out for maybe 3 hours now If I m using generics properly then `as` and `as ` are broken when used with generics

ClaudiuB · Answer

``` infix fun lt PossibleSuperClass gt KClass lt out Any gt instanceof clazz PossibleSuperClass Boolean where PossibleSuperClass KClass lt out Any gt return try this as PossibleSuperClass true catch e ClassCastException false ``` Same happens if i go the `as ` way My hunch is there s issues with `PossibleSuperClass` When debugging `this` `$receiver` is `java lang Void` `clazz` is `kotlin Number` and `this as PossibleSuperClass` returns `this`s value `java lang void` in this case The value of `this` is returned every time even when an exception should be thrown or `null` should be returned

cedric · Answer

Probably a simple consequence of erased generics

ClaudiuB · Answer

Can I unerase it them

cedric · Answer

Somewhat with `reified` But the code you pasted above is scary

kevinmost · Answer

what are you trying to check there `Nothing class is Number class` wouldn t even work

kevinmost · Answer

you could do `Nothing class is Number` which will return false because you re checking if a `KClass` object is an instance of `Number` which it isn t

ClaudiuB · Answer

I m just trying to check if a class a is a subclass of b

ClaudiuB · Answer

reverse behavior of java class isAssignableFrom

kevinmost · Answer

you can t do that with `is` or `instanceof` In Java that s something like `Foo class isAssignableTo Bar class ` I believe

cedric · Answer

Are you working with classes or values If the former `isAssignableFrom` is probably what you want

kevinmost · Answer

assignableFrom yeah

ClaudiuB · Answer

I wanted a java agnostic way of doing this disappointed

kevinmost · Answer

KClass should have a method for that probably

cedric · Answer

If you want the reverse of `isAssignableFrom` just reverse the arguments

ClaudiuB · Answer

I want it but kotlin code only I was at the `isAssignableFrom` an hour and something ago

cedric · Answer

`value class java isAssignableFrom `

kevinmost · Answer

hm yeah I don t see anything on `KClass` like `isAssignableFrom`

ClaudiuB · Answer

Think I found a way kotlin only

ClaudiuB · Answer

let me run the debugger

cedric · Answer

Just run the REPL

ClaudiuB · Answer

``` infix fun lt PossibleSuperClass gt KClass lt out Any gt instanceof clazz PossibleSuperClass Boolean where PossibleSuperClass KClass lt out Any gt return clazz nestedClasses contains this ```

ClaudiuB · Answer

oh wait maybe that s something else thought it was a metaphorical `nestedClass` like nested inside the inheritance hierarchy of the class

kevinmost · Answer

that isn t what you want yeah

ClaudiuB · Answer

``` infix fun lt PossibleSuperClass gt KClass lt out Any gt instanceof clazz PossibleSuperClass Boolean where PossibleSuperClass KClass lt out Any gt return clazz isInstance this ```

kevinmost · Answer

are you expecting `Int class instanceof Number class` to return true there

ClaudiuB · Answer

But if you end up processing classes in a programmatic way you can t use `as` or `as ` anymore

ClaudiuB · Answer

assuming `Int Number` i hope so

kevinmost · Answer

it won t because you re asking if `KClass lt Number gt ` is an instance of `Int`

kevinmost · Answer

`Int class isInstance 3 ` would be true

kevinmost · Answer

`Int class isInstance Number class ` would not

kevinmost · Answer

`KClass class isInstance Number class ` would be true

kevinmost · Answer

because `Number class` is a `KClass lt Number gt ` which makes it an instance of `KClass`

ClaudiuB · Answer

ok but then there isn t a way I guess

kevinmost · Answer

you could manually write a function that checks `superTypes` recursively maybe

kevinmost · Answer

There s `allSupertypes` that flattens out the hierarchy for you into one list but it s JVM only haha

ClaudiuB · Answer

Hmm I ll try with `superTypes` thanks for the idea Maybe I can figure it out without JVM after all

ClaudiuB · Answer

Thanks for the idea found out about `KClass starProjectedType` which returns `KType` which has `isSubtypeOf` Method is ``` infix fun lt PossibleSuperClass gt KClass lt out Any gt instanceof clazz PossibleSuperClass Boolean where PossibleSuperClass KClass lt out Any gt return this starProjectedType isSubtypeOf clazz starProjectedType ```

ClaudiuB · Answer

And the checks <https i imgur com uLNxG3Z png>

mwerschy · Answer

What s the point of doing this in pure Kotlin Neither of the other targets support the required reflection mechanisms so you ll be bound to the JVM anyway

ClaudiuB · Answer

Won t Kotlin ship with it s own VM eventually

ClaudiuB · Answer

VM that will hopefully implement natively the api at the time

mwerschy · Answer

Haven t seen anyone say that s a goal Source

mwerschy · Answer

Pretty sure someone from Kotlin Native explicitly said they aren t looking to add their own VM to that so would have to be a new target

ClaudiuB · Answer

Hmm okay maybe `Kotlin Native` is what I meant forget about the VM thing But Kotlin Native would still have to implement stuff like `KType` and its functions right

cedric · Answer

I m still not sure what you re trying to achieve that s not already covered by `is` and `as`

ClaudiuB · Answer

Or is `KType` a Kotlin class used as an umbrella thing just for Kotlin + JVM together

kevinmost · Answer

< cedric> he s trying to determine if one `KClass` instance is a supertype of another basically

ClaudiuB · Answer

Cedric if I try to use `x is y class` or through the type it complains if `y KType` and try `x is y` it complains it only works with Real Types like `OutOfMemoryException` not `platformUiException class`

kevinmost · Answer

`is` and `as` cover it when you know the types statically

ClaudiuB · Answer

yes I guess `static types` better describes it

kevinmost · Answer

these are dynamic class instances so if I m understanding his problem correctly it s exactly why you can t do something like ``` Class lt gt type = Foo class if myFoo instanceof type ``` in Java

cedric · Answer

Ah ok

kevinmost · Answer

unfortunately there s no platform agnostic `myKClass isAssignableFrom otherKClass ` you have to go through the ` java` extension and get a regular `Class` instance for that

ClaudiuB · Answer

The thing I posted seemed to work is that interracting with the JVM Only uses `KClass` and `KType`

ClaudiuB · Answer

``` e g object class instanceof Activity class infix fun lt PossibleSuperClass gt KClass lt out Any gt instanceof clazz PossibleSuperClass Boolean where PossibleSuperClass KClass lt out Any gt return this starProjectedType isSubtypeOf clazz starProjectedType e g object instanceof Activity class infix fun lt PossibleSuperClass gt Any instanceof clazz PossibleSuperClass Boolean where PossibleSuperClass KClass lt out Any gt return this class instanceof clazz ```

mwerschy · Answer

It relies on `starProjectedType` which is JVM only <https kotlinlang org api latest jvm stdlib kotlin reflect full star projected type html>

ClaudiuB · Answer

Alright but we have `KClass superTypes List lt KType gt ` which we could use instead thinking face Flatmap it to a boolean maybe

gildor · Answer

< ClaudiuB> Such thing that you trying to implement make sense but not sure that it will work on K N maybe you should ask in < C3SGXARS6|kotlin native> about real platform agnostic solution or just open an issue about this case