KeyMappedEnum.kt

interface EnumWithKey<T, K> {
    val T.key: K
}

/*
 * The reified type parameter lets you call the function without explicitly 
 * passing the Class-object.
 */
inline fun <reified T : Enum<T>, R> EnumWithKey<T, R>.byKey(key: R): T? {
    return enumValues<T>().find { it.key == key }
}

enum class Expr(val operator: String) {
    SUM("+"),
    MINUS("-"),
    // ...
    ;

    companion object : EnumWithKey<PaymentMethodType, String> {
        // Just define what the key is
        override val Expr.key
            get() = operator
    }
}

// The extension function byKey could be invoked like this:
Expr.byKey("+")