Custom Exception + Sealed Result Pattern

sealed class AppError(message: String) : Exception(message) {
    class NotFound(resource: String, id: Any)     : AppError("$resource not found: $id")
    class InvalidInput(field: String, why: String) : AppError("invalid $field: $why")
    class Unauthorized(msg: String = "unauthorized") : AppError(msg)
    class Conflict(what: String)                   : AppError("$what already exists")
}

interface UserRepo {
    fun find(id: Int): String
}

class UserService(private val repo: UserRepo) {
    fun greet(id: Int): String {
        if (id <= 0) throw AppError.InvalidInput("id", "must be positive")
        return runCatching { repo.find(id) }
            .getOrElse { throw AppError.NotFound("user", id) }
    }
}

fun handle(action: () -> Unit) {
    try { action() }
    catch (e: AppError) {
        when (e) {
            is AppError.NotFound     -> println("404: ${e.message}")
            is AppError.InvalidInput -> println("400: ${e.message}")
            is AppError.Unauthorized -> println("401: ${e.message}")
            is AppError.Conflict     -> println("409: ${e.message}")
        }
    }
}