KotlinNullableManagementExample.kt

sealed class Row {
    data class Header(
            val columns: List<String>
    ) : Row()

    data class  Item(
            val id: Long,
            val name: String,
            val price: Long?
    ) : Row()
}

interface  Parser {
    fun parse(content: String): List<Row>
    class IllegalFormatException(override  val message: String) : Exception(message)
}

class CSVParser : Parser {
    override fun parse(content: String): List<Row> {
        val rows = content.split(System.getProperty("line.separator"))
        val header = rows.getOrNull(0)
                ?.let { Row.Header(it.split(",")) }
                ?: throw Parser.IllegalFormatException("no header at row 1")

        val items = rows.drop(1)
                .mapIndexed { index, s ->
                    val l = s.split(",")
                    val id = l.getOrThrow(0, { "there are no 'id' at ${index + 1}"})
                            .toLongOrThrow { "${index + 1} is not a number: $it" }
                    val name = l.getOrThrow(1, {"there are not 'name' at ${index + 1} "})
                    val price = l.getOrNull(2)?.toLongOrNull()
                    Row.Item(id, name, price)
                }
        return listOfNotNull(header) + items
    }

}

private inline  fun List<String>.getOrThrow(index: Int, lazyMessage: () -> String): String {
        return this.getOrElse(index, { throw Parser.IllegalFormatException(lazyMessage())})
}

private inline  fun String.toLongOrThrow(radix: Int = 10, lazyMessage: (String) -> String): Long {
    try {
        return toLong(radix)
    } catch (e: NumberFormatException) {
        throw Parser.IllegalFormatException(lazyMessage(this))
    }
}