Get name of function argument in higher-order function

Question

It seems that Kotlin does not support getting local functions etc. Please see the example below:

fun <T> MutableList<T>.filterOnCondition(condition: (T) -> Boolean): MutableList<T>
{ 
    // For the printline, trying to get the function name, 
    // which in this case is passedAsCondition
    println(condition.reflect()!!.instanceParameter!!.name)
}

fun passedAsCondition (number: Int, multipleOf : Int): Boolean
{
    return number % multipleOf == 0
}

numbers.filterOnCondition { passedAsCondition(it, 5) }

Kotlin returns this error as it has not been mapped out yet:

"kotlin.reflect.jvm.internal.KotlinReflectionInternalError: Introspecting local functions, lambdas, anonymous functions and local variables is not yet fully supported in Kotlin reflection"

(https://github.com/JetBrains/kotlin/blob/master/core/reflection.jvm/src/kotlin/reflect/jvm/internal/EmptyContainerForLocal.kt#L41)

BUT, surely this is possible to do via Java, is it not?

Answer 1

It's an anonymous function, thus it's name will be <anonymous>:

val x: (Int) -> Boolean = { passedAsCondition(it, 5) }
println(x.reflect()?.name) //prints <anonymous>

When you have a lambda { passedAsCondition(it, 5) } how would you expect the reflection to work here? passedAsCondition is a simple call made inside the lambda, but you're invoking the reflect on an unnamed, anonymous, lambda, which does not have a name.

Ordinary functions can be used with method references which of course do have a name:

fun x(a: Int): Boolean {
    passedAsCondition(a, 5)
    return true
}
println(::x.name) //gives x

As a result, making use of proper reflection, the following works:

fun main(args: Array<String>) {
    mutableListOf(1).filterOnCondition(::passedAsCondition)
}


fun <T> MutableList<T>.filterOnCondition(
    condition: KFunction2<@ParameterName(name = "number") Int, @ParameterName(name = "multipleOf") Int, Boolean>
) {
    println(condition.name)
}

fun passedAsCondition(number: Int, multipleOf: Int): Boolean = number % multipleOf == 0