HList TupleConverter for Scalding

fun_with_scalding_and_shapeless.scala

import shapeless._
import shapeless.ops.hlist._
import shapeless.ops.nat._

import cascading.tuple.{Tuple, TupleEntry}

import com.twitter.scalding._

// documentation for cascading:
//   http://docs.cascading.org/cascading/2.5/javadoc/cascading/tuple/TupleEntry.html
//   http://docs.cascading.org/cascading/2.5/javadoc/cascading/tuple/Tuple.html

// documentation for scalding:
//   https://twitter.github.io/scalding/index.html#com.twitter.scalding.TupleConverter
//   https://twitter.github.io/scalding/index.html#com.twitter.scalding.TupleGetter$
//   https://twitter.github.io/scalding/index.html#com.twitter.scalding.TupleSetter
      
implicit def baseHListConverter[H](implicit gH: TupleGetter[H]): TupleConverter[H :: HNil] =
  new TupleConverter[H :: HNil] {
    override def apply(te: TupleEntry): H :: HNil = gH.get(te.getTuple, te.size() - 1) :: HNil

    override def arity: Int = 1
  }

implicit def recursiveHListConverter[H, T <: HList]
  (implicit gH: TupleGetter[H], tailConverter: TupleConverter[T]): TupleConverter[H :: T] =
    new TupleConverter[H :: T] {
      override def apply(te: TupleEntry): H :: T =
        gH.get(te.getTuple, te.size() - arity) :: tailConverter(te)

      override def arity: Int = (tailConverter.arity + 1)
    }

implicit def hListSetter[L <: HList, N <: Nat]
  (implicit tl: ToList[L, Any], len: Length.Aux[L, N], ti: ToInt[N]):
  TupleSetter[L] =
    new TupleSetter[L] {

      override def apply(arg: L): Tuple = {
        val argList: List[Any] = arg.toList
        new Tuple(argList.asInstanceOf[List[Object]]:_*)
      }

      override def arity: Int = ti()
    }