I’m struggling understanding how to use in/out for...
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I’m struggling understanding how to use in/out for generic types, here is what I’m trying to do :
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interface Executor<T, U> {
    fun isExecutable(): (T) -> Boolean
    fun execute(): suspend (U) -> Unit

class MyClass<T, U> {

     val executors = mutableMapOf<String, Executor<T, U>>()

    fun register(eventName: String, executor: Executor<T, U>) {
        executors[eventName] = executor

    inline fun <reified V : U> register(executor: Executor<T, V>) {
        val eventName = V::class.toString()
        register(eventName, executor)
I get the following error from the IDE
Type mismatch. Required: Executor<T, U>, Found: Executor<T, V>
is a sealed class and
a subtype of that class. I did read the page about generic and “PECS” in the doc, but I still haven’t figured it out. Anyone knows what I need to change?
your design isn't type safe and there's no way to express it in Kotlin without unchecked casts
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object A : Executor<Any?, Int> {
    override fun isExecutable() = { true }
    override fun execute() = suspend {
        assert(it is Int)
val obj = MyClass<Any?, Any?>()
obj.register(A) // you want this function?
obj.executors.values.first().execute()("string") // this is allowed by the interface but cannot work
You can mark type parameters T and U as
but not