Kotlin: How to extend the enum class with an extension function

user_2805265 0
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I'm trying to extend enum classes of type String with the following function but am unable to use it at the call site like so: 

fun <T: Enum<String>> Class<T>.join(skipFirst: Int = 0, skipLast: Int = 0): String {
    return this.enumConstants
        .drop(skipFirst)
        .dropLast(skipLast)
        .map { e -> e.name }
        .joinToString()
}

MyStringEnum.join(1, 1);

What am I doing wrong here?

kotlin-extension kotlin enums
4 Answers

Use of ::class is a nasty workaround. I suggest you to look at enumValues<E> and enumValueOf<E> from stdlib and do the same way:

inline fun <reified E : Enum<E>> joinValuesOf(skipFirst: Int = 0, skipLast: Int = 0): String =
        enumValues<E>().join(skipFirst, skipLast)

@PublishedApi
internal fun Array<out Enum<*>>.join(skipFirst: Int, skipLast: Int): String =
        asList()
                .subList(skipFirst, size - skipLast)
                .joinToString(transform = Enum<*>::name)

Usage: joinValuesOf<Thread.State>()

user_30502490
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