What if we needed Luigi-style input/output flattening but in a type safe way?

Flattener.scala

package flattening

import scala.annotation.implicitNotFound

object Flattener {
  def flatten[A]: PartiallyAppliedFlattener[A] = new PartiallyAppliedFlattener

  final class PartiallyAppliedFlattener[A] {
    def apply[B](value: B)(implicit ev: Flattenable[A, B]): List[A] = ev(value)
  }
}

@implicitNotFound("Cannot flatten ${B} into List[${A}]")
trait Flattenable[+A, -B] {
  def apply(value: B): List[A]
}

object Flattenable {
  implicit def identityFlatten[A]: Flattenable[A, A] = value => List(value)

  implicit def listFlatten[A, B](implicit ev: Flattenable[A, B]): Flattenable[A, List[B]] =
    list => list.flatMap(ev.apply)

  implicit def mapFlatten[K, A, B](implicit ev: Flattenable[A, B]): Flattenable[A, Map[K, B]] =
    map => map.values.toList.flatMap(ev.apply)

  implicit def traversableOnceFlatten[A, B](implicit
    ev: Flattenable[A, B]
  ): Flattenable[A, TraversableOnce[B]] =
    traversable => traversable.flatMap(ev.apply).toList
}

object FlattenerDemo extends App {
  println(Flattener.flatten[String]("hello"))
  println(Flattener.flatten[String](List("he", "llo")))
  println(Flattener.flatten[Char](List("he".toList, "llo".toList)))
  println(Flattener.flatten[String](Map(1 -> "he", 2 -> "llo")))
  println(Flattener.flatten[String](List(Map(1 -> "he", 2 -> "llo"), Map(3 -> "!"))))
  println(Flattener.flatten[String](Map(1 -> List("he"), 2 -> List("llo", "!"))))
  println(Flattener.flatten[String](List(Vector("he"), Vector("llo", "!"))))
  // Next line fails to compile with error: Cannot flatten scala.collection.immutable.Set[Int] into List[String]
//  println(Flattener.flatten[String](Set(12)))
}