There is normal extension function
fun <T> List<T>.test() { }
fun String.test(){ }
and i can declare a variable with extension function type
val obj1 = fun String.(){ }
but can not declare a variable with generic extension function type
val obj2 = fun <T> List<T>.() {} //error
P.S. I don't want to use List<Any>, it is different from generic.
Can someone tell me how to solve it? Thanks!!
You cannot do it, same as you cannot assign regular (non-extension) generic function to a variable or instantiate a generic class without specifying the type parameter. Try to put yourself in the compiler's shoes: what would be the type of such variable? It would be generic as well, so it is only possible in another generic context.
fun <T> List<T>.test(){ }
val v1 = List::test // No
val v2 = List<String>::test // OK
fun <T> other() {
val v3 = List<T>::test // OK
val v4 = fun List<T>.() { } // OK
}
Extension functions are kind of a red herring. Values can't have polymorphic types at all, not just in Kotlin, but in Java, Scala, etc. Depending on why you want it, one approach would be a generic function returning an extension function:
fun <T> obj2Generic() = fun List<T>.() {}
val obj2ForInt = obj2Generic<Int>()
val obj2ForString = obj2Generic<String>()
