Why are annotations in higher-order functions unsupported?

Question

This is the error I get when using custom annotation in a higher-order function:

Is there any way to use annotations in higher-order functions? If not, what would be an alternative solution (apart from using enums)?

This is how my custom annotation looks like:

companion object {
   private const val PERMISSION_DENIED = 1
   private const val PROVIDER_DISABLED = 2
   private const val SUCCESS = 3

   @IntDef(PERMISSION_DENIED, PROVIDER_DISABLED, SUCCESS)
   @Retention(AnnotationRetention.SOURCE)
   annotation class PreconditionResult
}

Answer 1

Workaround

There's a workaround which involves functional interfaces (notice the fun interface below).

Note: Unfortunately, functional interfaces that are defined in kotlin code are only available since the upcoming 1.4 release. You can test it now with 1.4-M1 .

fun interface FulFilled {
    fun execute(@PreconditionResult fulFilled: Int): Unit
}

private fun checkPrecondition(context: Context, fulFilled: Fulfilled) {
}

checkPrecondition(context) { fulFilled -> println("Got $fulFilled")}

Possible solution

I'm not sure if it would achieve what you need to, but it's also possible to drop the argument name from your type definition:

private fun checkPrecondition(context: String, callback: (@PreconditionResult Int) -> Unit) {

You'll need to change the target of your annotation to a type:

    @Retention(AnnotationRetention.SOURCE)
    @Target(AnnotationTarget.TYPE)
    annotation class PreconditionResult