Old wine, new bottles: safely migrating/permuting/extending field values between case classes using shapeless 2.0.0's LabelledGeneric.

gistfile1.scala

// See http://stackoverflow.com/questions/23192760/using-a-variable-as-an-argument-in-copy-on-a-case-class

import shapeless._, record._, syntax.singleton._, ops.hlist.Remove

/**
 * This will be in shapeless 2.1.0 ...
 *
 * Permute the elements of the supplied `HList` of type `L` into the same order as the elements of
 * the `HList` of type `M`.
 */
trait Align[L <: HList, M <: HList] extends (L => M) {
  def apply(l: L): M
}

object Align {
  import ops.hlist.Remove

  def apply[L <: HList, M <: HList](implicit alm: Align[L, M]): Align[L, M] = alm

  implicit val hnilAlign: Align[HNil, HNil] = new Align[HNil, HNil] {
    def apply(l: HNil): HNil = l
  }

  implicit def hlistAlign[L <: HList, MH, MT <: HList, R <: HList]
    (implicit select: Remove.Aux[L, MH, (MH, R)], alignTail: Align[R, MT]): Align[L, MH :: MT] = new Align[L, MH :: MT] {
    def apply(l: L): MH :: MT = {
      val (h, t) = l.removeElem[MH]
      h :: alignTail(t)
    }
  }
}

/**
 * This, or something like it, will be in shapeless 2.1.0 ...
 *
 * Utility trait intended for inferring a field type from a sample value and unpacking it into its
 * key and value types.
 */
trait Field {
  type K
  type V
  type F = FieldType[K, V]
}

object Field {
  def apply[K0, V0](sample: FieldType[K0, V0]) = new Field { type K = K0; type V = V0 }
}

object OldWineNewBottle {
  case class From(s1: String, s2: String)
  case class To(s2: String, i: Int, s1: String)

  val from = From("foo", "bar")

  val fromGen = LabelledGeneric[From]
  val toGen = LabelledGeneric[To]

  // Define the type of the i field by example
  val iField = Field('i ->> 0)

  val align = Align[iField.F :: fromGen.Repr, toGen.Repr]

  // extend the fields of From with a field for 'i', permute into
  // the correct order for To and create a new instance ...
  val to = toGen.from(align('i ->> 23 :: fromGen.to(from)))

  assert(to == To("bar", 23, "foo"))
}