KtorControllerAdapter.kt

@ContextDsl
fun Route.coGet(path: String, block: suspend ApplicationCall.() -> ServerResponse): Route = get("/") {
	val result = call.block()

	when (result) {
		is ServerResponse.Empty ->
			call.respondText { "" }

		is ServerResponse.Json ->
			call.respondText(result.jsonString, ContentType.Application.Json)

		is ServerResponse.Error ->
			call.respondText(ContentType.Application.Json, HttpStatusCode.fromValue(result.statusCode)) {
				logger.error{ "${result.message}: caused by ${result.exception}" }

				result.message
			}
	}
}

Another possible alternative (I guess):

@ContextDsl fun Route.get(path: String): Route = get(path) { } // Note: Lambda intentionally empty
@ContextDsl fun Route.post(path: String): Route = post(path) { } // Note: Lambda intentionally empty

sealed class ServerResponse {
    object Empty : ServerResponse()

    data class Error(val statusCode: Int, val message: String, val exception: Exception? = null) : ServerResponse()

    data class Json(val jsonObj: Any) : ServerResponse() {
        val jsonString: String
            get() = Klaxon().toJsonString(jsonObj)
    }
}

fun Route.coHandler(block: suspend ApplicationCall.() -> ServerResponse): Route = this.apply {
    handle {
        val result = call.block()

        when (result) {
            is ServerResponse.Empty ->
                call.respondText { "" }

            is ServerResponse.Json ->
                call.respondText(result.jsonString, ContentType.Application.Json)

            is ServerResponse.Error ->
                call.respondText(ContentType.Application.Json, HttpStatusCode.fromValue(result.statusCode)) {
                    logger.error { "${result.message}: caused by ${result.exception}" }

                    result.message
                }
        }
    }
}

fun Application.mymodule() {
    routing {
        get("/").coHandler {
            ServerResponse.Empty
        }
    }
}