Why scala's pattern maching does not work in for loops for type matching?

Question

I\'m coding against an API that gives me access to remote file system. The API returns a list of files and directories as list of node objects (parent to file and directory)

Answer 1

What you want to do is essentially: iterate over all elements of workarea.getChildren() that are of type CSDir (in other words: matching some criteria). Ordinary loop/for comprehension iterates over all elements. You cannot say: iterate over all elements having this type and skip others. You must be more explicit.

What do you think about:

workarea.getChildren() collect {case dir: CSDir => dir} foreach println

It does exactly what you want: collect all elements of workarea.getChildren() and for each of them call println.

Answer 2

How about this:

val listOfBaseObjects: List[Any] = List[Any]("a string", 1:Integer);

for (x <- listOfBaseObjects if x.isInstanceOf[String]) {
  println(x)
}

Answer 3

This is the long-standing issue 900 and has been discussed many times before. The common workaround is to use something like:

for (y@(_y:String) <- listOfBaseObjects) {
    println(y)
}

A nicer version is provided by Jason Zaugg in the comments to the above-mentioned ticket:

object Typed { def unapply[A](a: A) = Some(a) }

for (Typed(y : String) <- listOfBaseObjects) {
    println(y)
}