summon.kt

inline fun <reified K : Any> summon(): K {
    val tpe = typeOf<K>()
    return summon(tpe) as? K
        ?: error("Trait instance must be of type $tpe")
}

// FIXME: Come up with a way to make it sufficiently weak to prevent
//        memory leaks due to accumulation of the underlying classes
//        if class loaders are being closed.
private val instances = ConcurrentHashMap<KType, Any>()

fun summon(tpe: KType): Any {
    return instances.computeIfAbsent(tpe) {
        require(tpe.arguments.size == 1) {
            "Multi-parameter traits are not supported"
        }

        val arg = tpe.arguments[0].type?.classifier as? KClass<*>
            ?: throw IllegalArgumentException("Trait's type parameter must be a class")

        val companion = arg.companionObjectInstance
            ?: throw IllegalArgumentException("Trait's type parameter must have a companion object")

        val companionType = arg.companionObject
            ?: throw IllegalArgumentException("Trait's type parameter must have a companion object")

        val companionProperties = companionType.memberProperties

        val count = companionProperties.count { it.returnType == tpe }
        require(count == 1) {
            "Companion object of the trait's type parameter must have exactly one property of trait's type $tpe"
        }

        val field = companionProperties.find { it.returnType == tpe }
            ?: throw IllegalArgumentException("Companion object of the trait's type parameter must have a property of trait's type $tpe")

        field.getter.call(companion)
            ?: throw IllegalArgumentException("Trait instance must not be null")
    }
}