Can anyone explain what I'm doing wrong here? ``` ...

# announcements

[jeggy]

Can anyone explain what I'm doing wrong here?

fun <T> test(list: List<T>) = when (list) {
    is List<String> -> println("Hello World")
    is List<List<Int>> -> println("Deep")
    else -> println("No Clue")
}

[diesieben07]

Generics are erased. At runtime there is only List, so this check is not possible.

[ribesg]

Let's say I compile your code then decompile it, here's what it would look like:

fun test(list: List) = when (list) {
    is List -> println("Hello World")
    is List -> println("Deep")
    else -> println("No Clue")
}

[Shawn]

that should be a compile error, too, Cannot check for instance of erased type

[jeggy]

alright thanks 🙂

[jeggy]

how would you solve something like this?

[Egor Trutenko]

You might want to reify type argument

[Egor Trutenko]

https://kotlinlang.org/docs/reference/inline-functions.html#reified-type-parameters

[elizarov]

data class ListOfStrings(val list: List<String>) & etc — use those instead of plain lists & you’ll be able to check their types.

[Shawn]

reified won’t make the is List<T> check valid - you can match against T::class, but that would likely fail against the List<List<Int>> case

[terorie]

It’s a very bad approach but you could cast list to List<Any>, then check if list[0] is String/List