gistfile1.txt

[info] Compiling 9 Scala sources to /Users/ian/proj/macalinao/scala-playground/target/scala-2.12/classes...
[error] /Users/ian/proj/macalinao/scala-playground/src/main/scala/pw/ian/playground/shapeless/KittensMonoidDerivation.scala:33: value dragon is not a member of type parameter Wrapper
[error]       val groups = al.groupBy(_.dragon) |+| bl.groupBy(_.dragon)
[error]                                 ^
[error] /Users/ian/proj/macalinao/scala-playground/src/main/scala/pw/ian/playground/shapeless/KittensMonoidDerivation.scala:33: type mismatch;
[error]  found   : cats.kernel.Monoid[Map[Nothing,List[Wrapper]]]
[error]  required: cats.Semigroup[scala.collection.immutable.Map[K,List[Wrapper]]]
[error]     (which expands to)  cats.kernel.Semigroup[scala.collection.immutable.Map[K,List[Wrapper]]]
[error]       val groups = al.groupBy(_.dragon) |+| bl.groupBy(_.dragon)
[error]                              ^
[error] /Users/ian/proj/macalinao/scala-playground/src/main/scala/pw/ian/playground/shapeless/KittensMonoidDerivation.scala:33: value |+| is not a member of scala.collection.immutable.Map[K,List[Wrapper]]
[error]       val groups = al.groupBy(_.dragon) |+| bl.groupBy(_.dragon)
[error]                                         ^
[error] /Users/ian/proj/macalinao/scala-playground/src/main/scala/pw/ian/playground/shapeless/KittensMonoidDerivation.scala:33: value dragon is not a member of type parameter Wrapper
[error]       val groups = al.groupBy(_.dragon) |+| bl.groupBy(_.dragon)
[error]                                                          ^
[error] four errors found
[error] (compile:compileIncremental) Compilation failed
[error] Total time: 2 s, completed May 20, 2017 2:02:43 AM
>

KittensMonoidDerivation.scala

package pw.ian.playground.shapeless

import cats._
import cats.implicits._
import cats.data.NonEmptyList
import cats.derived._, monoid._, legacy._

// runMain "pw.ian.playground.shapeless.KittensMonoidDerivation"
object KittensMonoidDerivation {

  trait Dragon
  case object Infernal extends Dragon

  case class Moments(ct: Int, sum: Int, sumSq: Int)

  case class Wrapper(dragon: Dragon, data: Option[Moments])

  case class All(wrappers: Seq[Wrapper])

  implicit val momentsMonoid: Monoid[Moments] = Monoid[Moments]

  implicit val wrapperSemi = new Semigroup[Wrapper] {
    def combine(a: Wrapper, b: Wrapper): Wrapper = {
      a.copy(data = a.data |+| b.data)
    }
  }

  implicit def sortedSeqMonoid[Wrapper]: Monoid[Seq[Wrapper]] = new Monoid[Seq[Wrapper]] {

    def combine(a: Seq[Wrapper], b: Seq[Wrapper]): Seq[Wrapper] = {
      val al = a.toList
      val bl = b.toList
      val groups = al.groupBy(_.dragon) |+| bl.groupBy(_.dragon)
      groups
        .mapValues(x => NonEmptyList.fromList(x).map(_.reduce))
        .values.flatten.toList
    }

    def empty = List()

  }

  def main(args: Array[String]): Unit = {

    val a = All(
      wrappers = Seq(
        Wrapper(
          dragon = Infernal,
          data = Moments(1, 2, 3).some,
        ),
      ),
    )
    val b = All(
      wrappers = Seq(
        Wrapper(
          dragon = Infernal,
          data = Moments(4, 5, 6).some,
        ),
      ),
    )

    println(a |+| b)
  }

}