Code for Travis Brown's answer to State Transformations with a Shapless State Monad (http://stackoverflow.com/questions/34870889/state-transformations-with-a-shapeless-state-monad)

shapeless-transformations.scala

package ss

object ContrivedAdd {

  import shapeless._
  import record._
  import syntax.singleton._

  import scalaz._
  import Scalaz._

  def transform[S, T](f: S => T): IndexedState[S, T, Unit] =
    IndexedState(s => (f(s), ()))

  def addField[K <: Symbol, A, L <: HList](k: Witness.Aux[K], a: A)(
    f: Int => Int
  ): IndexedState[S[L], S[FieldType[K, A] :: L], Unit] =
    IndexedState(s => (S(f(s.total), field[K](a) :: s.scratch), ()))

  case class S[L <: HList](total: Int, scratch: L)

  def contrivedAdd[L <: HList](n: Int) = for {
    a <- init[S[L]]
    _ <- addField('latestAdded, n)(_ + n)
    r <- get
  } yield r.total

  def main(args: Array[String] = Array()): Unit = {
    val initialState = S(0, ('someDefault ->> 1) :: HNil)
    val result = contrivedAdd(555).run(initialState)
    println(result)
  }
}