either

sealed interface AppError {
    data class ResourceNotFound(val id: String): AppError
    data class ResourceForbidden(val id: String): AppError
    data class UserError(val messageCodes: List<String>): AppError
}

fun AppError.toResponse(): Response = when(this) {
  is ResourceNotFound -> Response(Status.NOT_FOUND)
  is ResourceForbidden -> Response(Status.FORBIDDEN)
  is UserError -> Response(Status.BAD_REQUEST)
}.body(marshaller.asFormatString(this))

routes += "/v1/things" / idLens bindContract GET to { id ->
  { req ->
    service.getThing(authLens(req), id)
      .map { Response(Status.OK).with(thingLens of it) }
      .recover(AppError::toResponse)
  }
}