I think we can rewrite this a bit to make it more clear:

```scala
val parametersConstraints = (route.call.parameters.getOrElse(Nil).collect {
  case p if localNames.contains(p.name) && p.fixed.isDefined => s"${p.name} == ${p.fixed.get}"
}) match {
  case Seq() => ""
  case nonEmpty => s"if ${nonEmpty.mkString(" && ")}"
}