Merging arbitrary case classes via shapeless ... (LabelledGeneric and record merge).

gistfile1.scala

import shapeless._

object CaseClassMergeDemo extends App {
  import mergeSyntax._

  case class Foo(i: Int, s: String, b: Boolean)
  case class Bar(b: Boolean, s: String)

  val foo = Foo(23, "foo", true)
  val bar = Bar(false, "bar")

  val merged = foo merge bar
  assert(merged == Foo(23, "bar", false))
}

// Implementation in terms of LabelledGeneric ...
object mergeSyntax {
  implicit class MergeSyntax[T](t: T) {
    def merge[U](u: U)(implicit merge: CaseClassMerge[T, U]): T = merge(t, u)
  }
}

trait CaseClassMerge[T, U] {
  def apply(t: T, u: U): T
}

object CaseClassMerge {
  import ops.record.Merger

  def apply[T, U](implicit merge: CaseClassMerge[T, U]): CaseClassMerge[T, U] = merge

  implicit def mkCCMerge[T, U, RT <: HList, RU <: HList]
    (implicit
      tgen: LabelledGeneric.Aux[T, RT],
      ugen: LabelledGeneric.Aux[U, RU],
      merger: Merger.Aux[RT, RU, RT]
    ): CaseClassMerge[T, U] =
    new CaseClassMerge[T, U] {
      def apply(t: T, u: U): T =
        tgen.from(merger(tgen.to(t), ugen.to(u)))
    }
}

Is there a way to write a merger that just ignores fields of the rhs class that are not present in the lhs class? That is, make the following example work:

case class Foo(a: String, b: Int, c: String)

case class Bar(b: Int, d: String)

val foo = Foo("hello", 42, "world")
val bar = Bar(1, "bar")

val merged = foo merge bar
assert(merged == Foo("hello", 1, "world"))

@lambdista Did you ever figure out how to do what you're asking?

No, to tell the truth I asked out of curiosity when I started studying shapeless but didn't need to accomplish that task so I didn't delve into accomplishing that.

No worries. Thanks for getting back to me and not pulling a https://xkcd.com/979/.

lol, you're welcome!