package swag

import com.typesafe.config.Config
import shapeless._
import shapeless.labelled._


object FromConfig {
  sealed trait Error

  //case class FieldMissing(fieldName: String) extends Error
  //case class WrongType(fieldName: String) extends Error

  def instance[A](f: String => Config => A) = new FromConfig[A] {
    override def read(path: String)(implicit conf: Config): A = f(path)(conf)
  }

  implicit val strConfig: FromConfig[String] = instance(path => conf => conf.getString(path) )
  implicit val intConfig: FromConfig[Int] = instance(path => conf => conf.getInt(path) )
  implicit val fromConfigHNil: FromConfig[HNil] = instance(path => conf => HNil)

  implicit def fromConfigHlist[K <: Symbol, H, T <: HList](implicit witness: Witness.Aux[K],
                                        hDec: FromConfig[H],
                                        tDec: FromConfig[T])
  : FromConfig[FieldType[K, H] :: T] = instance { path => implicit conf =>
    val hName = witness.value.name
    val h = field[K](hDec.read(path + "."  + hName))
    val t = tDec.read(path)
    h :: t
  }

  implicit def fromConfigGen[A,B](implicit gen: LabelledGeneric.Aux[A, B],
                                    bDec: FromConfig[B]): FromConfig[A] = instance { path => implicit conf =>
    gen.from(bDec.read(path))
  }

  def read[A: FromConfig](root: String, conf: Config): A =
    implicitly[FromConfig[A]].read(root)(conf)

}


trait FromConfig[A] {
  def read(path: String)(implicit conf: Config): A
}