Doobie readWithDefaults

readWithDefaults.scala

/**
  * Summon `Read[A]` instance for case class `A` with default parameters.
  * {{{
  *   case class A(a: String, b: Boolean = false)
  *   implicit val aRead: Read[A] = readWithDefaults[A]()
  * }}}
  */
class readWithDefaults[A] {

  def apply[
    Repr <: HList,
    Defaults <: HList,
    ZippedIn <: HList,
    WithDefaults <: HList,
    ZippedOut <: HList,
    Result <: HList
  ]()(implicit
    generic: Generic.Aux[A, Repr],
    defaults: Default.Aux[A, Defaults],
    zipperIn: Zip.Aux[Repr :: Defaults :: HNil, ZippedIn],
    mapperIn: Mapper.Aux[readWithDefaults.MapIn.type, ZippedIn, WithDefaults],
    read: Lazy[Read[WithDefaults]],
    zipperOut: Zip.Aux[WithDefaults :: Defaults :: HNil, ZippedOut],
    mapperOut: Mapper.Aux[readWithDefaults.MapOut.type, ZippedOut, Result],
    ev: Result =:= Repr,
  ): Read[A] = {
    val (_, _) = (zipperIn, mapperIn) // to prevent unused warning
    read.value.map { withDefaults =>
      generic.from((withDefaults zip defaults()).map(readWithDefaults.MapOut))
    }
  }
}
object readWithDefaults {
  def apply[A]: readWithDefaults[A] = new readWithDefaults[A]

  private object MapIn extends Poly1 {
    implicit def withDefault[A]: Case.Aux[(A, Some[A]), Option[A]] =
      at { case (a, _) =>
        Some(a)
      }

    implicit def withoutDefault[A]: Case.Aux[(A, None.type), A] =
      at { case (a, _) =>
        a
      }
  }

  private object MapOut extends Poly1 {
    implicit def withDefault[A]: Case.Aux[(Option[A], Some[A]), A] =
      at { case (value, Some(defaultValue)) =>
        value.getOrElse(defaultValue)
      }

    implicit def withoutDefault[A]: Case.Aux[(A, None.type), A]   =
      at { case (value, _) =>
        value
      }
  }
}

tpolecat/doobie#274