Considering this code is it possible to filter a l...

# getting-started

[igor.wojda]

Considering this code is it possible to filter a list using reified type .customFilterReified<AdvancedInterface>()
and return a list of Advanced objects instead of BaseInterface objects (heaving same behaviour as filtering using KClass
.customFilterWithKClass(AdvancedInterface::class)) ? 🤔

interface BaseInterface
interface AdvancedInterface

class Advanced : BaseInterface, AdvancedInterface {
    val someProperty = "someProperty"
}

// 1 FILTER USING KCLASS - OK
fun <T : BaseInterface> List<T>.customFilterWithKClass(item: KClass<*>): List<T> = filter { item.isInstance(it) }

// 2 FILTER USING REIFIED - PROBLEM
inline fun <reified T> List<BaseInterface>.customFilterReified(): List<BaseInterface> = filter { it is T }

// USAGE
fun main() {
    val items = listOf(Advanced())

    // 1 FILTER USING KCLASS - OK
    items // List<Advanced>
        .customFilterWithKClass(AdvancedInterface::class) // returns List<Advanced> - OK
        .map { it.someProperty } //

    // 2 FILTER USING REIFIED - PROBLEM
    items // List<Advanced>
        .customFilterReified<AdvancedInterface>() // returns List<BaseInterface> - PROBLEM
        .map { it.someProperty } // ERROR: Unresolved reference: someProperty
}

[Wout Werkman]

You forgot to actually use your reified generic argument in your function signature. If you would write:

inline fun <reified T> List<BaseInterface>.customFilterReified(): List<T /* Not BaseInterface */> = …

I’m quite sure that what you want is items.filterIsInstance<AdvancedInterface>()

[Wout Werkman]

Also, if you use `.mapNotNull { it as? T }`you wouldn’t need an unsafe cast

[igor.wojda]

Good catch. I have updated the code a bit using you suggestions, however the customFilterReified gives me AdvancedInterface type rather than Advanced type. Is there a way to get the oriiginal receiver type within the extension (Advanced) 🤔

interface BaseInterface
interface AdvancedInterface

class Advanced : BaseInterface, AdvancedInterface {
    val someProperty = "someProperty"
}

// 1 FILTER USING KCLASS - OK
fun <T : BaseInterface> List<T>.customFilterWithKClass(item: KClass<*>): List<T> = filter { item.isInstance(it) }

// 2 FILTER USING REIFIED - PROBLEM
inline fun <reified T> List<BaseInterface>.customFilterReified(): List<T> = filterIsInstance<T>()

// USAGE
fun main() {
    val items = listOf(Advanced())

    // 1 FILTER USING KCLASS - OK
    items // List<Advanced>
        .customFilterWithKClass(AdvancedInterface::class) // returns List<Advanced> - OK
        .map { it.someProperty } //

    // 2 FILTER USING REIFIED - PROBLEM
    items // List<Advanced>
        .customFilterReified<AdvancedInterface>() // returns List<AdvancedInterface> - PROBLEM
        .map { it.someProperty } // ERROR: Unresolved reference: someProperty

}

[Wout Werkman]

Well, you'd have to provide a more realistic example.
In your case this is actually a problem with customFilterWithKClass. It should be KClass<T>. Currently it just infers T from the receiver list. So the only reason // 1 works is because val items is already of type Advanced.
There is no realistic use case to call List<A>.filterIsInstance<B>() given that B is a supertype of A. And your compiler output is exactly as you should expect