Configuration loading with Typesafe config in Kotlin

Config.kt

package com.mantono

/**
 * implementation("com.typesafe:config:1.4.0")
 * See https://github.com/lightbend/config
 */

import com.typesafe.config.ConfigFactory
import com.typesafe.config.Config as TypeSafeConfig

object Config: Map<String, String> by loadProperties()

fun Map<String, String>.forContext(context: String): Map<String, String> =
    this.asSequence()
        .filter { it.key.startsWith(context) }
        .map { it.key.removePrefix("$context.") to it.value }
        .toMap()

private fun loadProperties(): Map<String, String> {
    val env = Environment()
    val config = ConfigFactory.load().forEnvironment(env)
    return config.entrySet().asSequence()
        .map { it.key to it.value.unwrapped().toString() }
        .toMap()
}

private fun TypeSafeConfig.forEnvironment(env: Environment): TypeSafeConfig {
    val envKeyPrefix: String = env.name.toLowerCase()
    return if(hasPath(envKeyPrefix)) {
        this
            .getConfig(env.name.toLowerCase())
            .withFallback(this)
    } else {
        this
    }
}

/**
 * Check if an environment variable is present of if a configuration value is set in application.conf,
 * or similar file.
 * Config key `jwk.issuer` will become environment variable `JWK_ISSUER`.
 * Any environment variable set will take presedence over a value present in a configuration file.
 */
fun getConfig(configKey: String): String {
    val envVarKey: String = configKey.toUpperCase().replace(".", "_")
    val envVarValue: String? = System.getenv(envVarKey)
    val configValue: String? = Config[configKey]
    return envVarValue ?: configValue
    ?: throw IllegalStateException("No value found for environment variable '$envVarValue' or config key '$configKey'")
}

enum class Environment {
    PRODUCTION,
    DEVELOPMENT,
    STAGING,
    TEST;

    companion object {
        operator fun invoke(env: String = System.getenv().getValue("APP_ENV")): Environment {
            return when(val appEnv = env.trim().toUpperCase()) {
                "TEST" -> TEST
                "DEV" -> DEVELOPMENT
                "STAGE" -> STAGING
                "PROD" -> PRODUCTION
                else -> valueOf(appEnv)
            }
        }
    }
}