CsvShapeless.scala

package csvShapeless

import shapeless._, labelled.{ field, FieldType }, syntax.singleton._

import scala.util.{Try,Success,Failure}


case class Person(name: String, surname: String, age: Int)

object Main extends App {

  import CSVConverter._

  val hListConverter = CSVConverter[String :: String :: Int :: HNil]

  println(hListConverter.from("mario,pastorelli,28")) // works

  // val converter = CSVConverter[Person] // doesn't because Coproduct is missing
}


class CSVException(s: String) extends RuntimeException

trait CSVConverter[T] {
  def from(s: String): Try[T]
  def to(t: T): String
}

object CSVConverter {
  def apply[T](implicit st: Lazy[CSVConverter[T]]): CSVConverter[T] = st.value

  def fail(s: String) = Failure(new CSVException(s))

  // Primitives

  implicit def stringCSVConverter: CSVConverter[String] = new CSVConverter[String] {
    def from(s: String): Try[String] = Success(s)
    def to(s: String): String = s
  }

  implicit def intCsvConverter: CSVConverter[Int] = new CSVConverter[Int] {
    def from(s: String): Try[Int] = Try(s.toInt)
    def to(i: Int): String = i.toString
  }


  //

  implicit def deriveHNil: CSVConverter[HNil] =
    new CSVConverter[HNil] {
      def from(s: String): Try[HNil] = s match {
        case "" => Success(HNil)
        case s => fail("Cannot convert " ++ s ++ " to HNil")
      }
      def to(n: HNil) = ""
    }
  
  implicit def deriveHCons[V, T <: HList]
    (implicit scv: Lazy[CSVConverter[V]], sct: Lazy[CSVConverter[T]])
        : CSVConverter[V :: T] =
      new CSVConverter[V :: T] {

        def from(s: String): Try[V :: T] = s.span(_ != ',') match {
          case (before,after) =>
            for {
              front <- scv.value.from(before)
              back <- sct.value.from(if (after.isEmpty) after else after.tail)
            } yield front :: back

          case _ => fail("Product miss = " ++ s)
        }

        def to(ft: V :: T): String = {
          scv.value.to(ft.head) ++ "," ++ sct.value.to(ft.tail)
        }
      }
}