Prisms for scalapb, as `oneof` is tagged union

pbPrism.scala

import scalapb.GeneratedOneof
import scalapb.lenses.{Lens, Mutation}

trait Prism[OneOf, Case] {
  def down(o: OneOf): Option[Case]
  def up(c: Case): OneOf
}

implicit class OneOfLens[A, OneOf <: GeneratedOneof](l: Lens[A, OneOf]) {
  def mutateCase[B](p: Lens[A, OneOf] => Prism[OneOf, B])(m: Lens[B, B] => Mutation[B]): Mutation[A] = a => {
    val prism = p(l)
    val oneOf = l.get(a)
    prism.down(oneOf) match {
      case Some(b) =>
        val mutation = m(Lens.unit)
        val newOneOf = prism.up(mutation(b))
        l.set(newOneOf)(a)
      case None => a
    }
  }
}