List elements matching on their type

Question

I've got a code similar to one below:

def walkTree(list:List[Command]) {
    list match {
        case Command1::rest => doSomething(); walkTree(rest)
        case Command2::rest => doSomethingElse(); walkTree(rest)
        case Nil => ;
    }
}

I also know that you can pattern match on specific type and assign a variable at the same time:

try {
    ...
}
catch {
    case ioExc:IOException => ioExc.printStackTrace()
    case exc:Exception => throw new RuntimeException("Oh Noes", e);
}

Is there a way to combine both in something like below:

def walkTree(list:List[Command]) {
    list match {
        case cmd1:Command1::rest => doSomething(); walkTree(rest)
        case cmd2:Command2::rest => doSomethingElse(); walkTree(rest)
        case Nil => ;
    }
}

Or do I need to extract each list element before matching?

Answer 1

Yes, just use parentheses like this (see example below):

def walkTree(list:List[Command]) {
    list match {
        case (cmd1:Command1)::rest => doSomething(); walkTree(rest)
        case (cmd2:Command2)::rest => doSomethingElse(); walkTree(rest)
        case Nil => ;
    }
}

However, can't you use foreach for this:

scala> List(A(1), B(2), A(3), B(4), A(5)).foreach(_ match {
     |     case (a:A) => println("a:" + a)
     |     case (b:B) => println("b:" + b)
     | })
a:A(1)
b:B(2)
a:A(3)
b:B(4)
a:A(5)

---

Example:

scala> case class A(val i: Int);
defined class A

scala> case class B(val i: Int);
defined class B

scala> def walkTree(list: List[ScalaObject]) {
     |     list match {
     |         case (a:A)::rest => println("a:" + a); walkTree(rest);
     |         case (b:B)::rest => println("b:" + b); walkTree(rest);
     |         case Nil => ;
     |     }
     | }
walkTree: (list: List[ScalaObject])Unit

scala> walkTree(List(A(1), B(2), A(3), B(4), A(5)))
a:A(1)
b:B(2)
a:A(3)
b:B(4)
a:A(5)

Answer 2

Using foreach and then pattern match on each element seems to be clearer for me:

def walkTree(list:List[Command]) {
  list foreach {
    _ match {
      case cmd1:Command1 => doSomething()
      case cmd2:Command2 => doSomethingElse()
      case _ =>
    }
  }
}