for (pat <- expr1) yield expr2

expr1 withFilter {
  case pat => true
  case _ => false
} map {
  case pat => expr2
}

val t = List("a", 1, 1.0)
for ((y : String) <- t) yield y

t withFilter {
  case y :String => true
  case _ => false
} map {
  case y : String => y
}