Typed environment config with validation errors

data class Config(val host: String, val port: Int, val debug: Boolean)

fun loadConfig(env: Map<String, String> = System.getenv()): Config {
    val host = env["APP_HOST"]?.trim().takeUnless { it.isNullOrEmpty() } ?: "0.0.0.0"
    val rawPort = env["APP_PORT"] ?: "8080"
    val port = rawPort.toIntOrNull() ?: error("APP_PORT must be an integer, got $rawPort")
    require(port in 1..65535) { "APP_PORT out of range: $port" }
    val debug = when (env["APP_DEBUG"]?.lowercase()) {
        null, "0", "false" -> false
        "1", "true" -> true
        else -> error("APP_DEBUG must be true/false")
    }
    return Config(host, port, debug)
}

val config = loadConfig(mapOf("APP_PORT" to "9090"))