Question

I have the following data model which I’m going to do pattern matching against later:

abstract class A
case class C(s:String) extends A

abstract class B extends A
case class D(i:Int) extends B
case class E(s:Int, e:Int) extends B

A is the abstract super type of the hierarchy. C is a concrete subclass of A. Other concrete subclasses of A are subclasses of B which is in turn a subclass of A.

Now if I write something like this, it works:

def match(a:A) a match {
   a:C => println("C")
   a:B => println("B")
}

However, in a for loop I cannot match against B. I assume that I need a constructor pattern, but since B is abstract, there is no constructor pattern for B.

val list:List[A] = List(C("a"), D(1), E(2,5), ...)

for (b:B <- list) println(b)  // Compile error
for (b@B <- list) println(b)  // Compile error

Here, I would like to print only B instances. Any workaround for this case?

Answer 1

You can use collect:

list.collect { case b: B => println(b) }

If you want to better undertand this, I recommend to read about partial functions. Here for example.