An HTML select element whose options & selected option are determined by a Laminar Signal of an arbitrary type T, emitting the selected T on a WriteBus

HtmlSelect.scala

import com.raquo.laminar.api.L.*
import org.scalajs.dom.{HTMLOptionElement, html}
 
object HTMLSelect {
  def signalRequiredSelect[T](options: Signal[List[T]], toValue: T => String, toText: T => String, controlledSelection: Signal[T], userSelection: WriteBus[T]): HtmlElement = {
    select(
      children <--
        options
          .split(toValue) { case (v, _, updates) =>
            option(
              value := v,
              child.text <-- updates.map(toText),
            )
          },
      onMountBind { ctx =>
        val ownedOptions: OwnedSignal[List[T]] = options.observe(ctx.owner)
        controlled(
          value <-- controlledSelection.map(toValue),
          onChange.mapToValue -->
            userSelection
              .contracomposeWriter[String](_.map(userSelectionValue => ownedOptions.now().find(toValue(_) == userSelectionValue).getOrElse(throw new Exception(s"Unexpected value '$value"))))(ctx.owner)
        )
      }
    )
  }

  def signalOptionalSelect[T](options: Signal[List[T]], toValue: T => String, toText: T => String, controlledSelection: Signal[Option[T]], userSelection: WriteBus[Option[T]], noSelectionValue: String = "noselection", noSelectionText: String = ""): HtmlElement =
    signalRequiredSelect[Option[T]](
      options.map(os => None :: os.map(Option(_))),
      _.fold(noSelectionValue)(toValue),
      _.fold(noSelectionText)(toText),
      controlledSelection,
      userSelection
    )
}