Scala compile warning with pattern matching

Question

I have an Akka Actor that has the following case pattern match check in its receive method as below:

def receive = {
  case x: (String, ListBuffer[String]) if(x._2.size >= 0) => {
  .....
  .....
}

When I compile, I get to see the following compiler warnings:

warning: non-variable type argument String in type pattern (String, scala.collection.mutable.ListBuffer[String]) 
is unchecked since it is eliminated by erasure)

Any clues as to how I could get rid of them? I do not want to set the compiler settings to ignore these warnings, but I do not see a reason why the compiler issues a warning?

Scala only sees from x during runtime that it is a Tuple2, so if a Tuple2 comes with different type parameters, that might be a real problem there. — Gábor Bakos, Mar 10 '14 at 17:15

score 3 · Answer 1 · answered Mar 10 '14 at 17:52

3

This is due to the JVM's type erasure. At runtime, the JVM only sees ListBuffer[Any]. Static type information of generics is lost. If you don't care about the generic type of the ListBuffer, you can change the pattern match to:

case x: (String, ListBuffer[_]) if(x._2.size >= 0) =>

answered Mar 10 '14 at 17:52

Dia Kharrat

5,948
3
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This would mean that I have to cast x._2 to a ListBuffer[String] before I could use it. Not so ideal! – joesan Mar 11 '14 at 08:14

score 2 · Accepted Answer · answered Mar 10 '14 at 17:55

2

One little trick I like to use for this problem is type aliasing.

type MyBuffer = ListBuffer[String]

//...

def receive = {
  case x: (String, MyBuffer) if(x._2.size >= 0) => {
  //.....
  //.....
}

answered Mar 10 '14 at 17:55

joescii

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Except this doesn't resolve the actual problem at all. – Alexey Romanov Mar 10 '14 at 20:27
@AlexeyRomanov Try it. The compiler will NOT throw the warning. He merely wants the warnings to go away. – joescii Mar 10 '14 at 20:27
The issue is that he didn't see the reason for the warning, see the last sentence of the question. – Alexey Romanov Mar 10 '14 at 20:35
@AlexeyRomanov Sure, I don't address the real issue or answer his last question. But if it truly is a "don't care", then this is another handy trick to get around it. Thanks for clearing it up. :) – joescii Mar 10 '14 at 20:37
I tried using a type alias, but the warning is still there! – joesan Mar 11 '14 at 08:12
Really? I'm surprised. I use it all the time. :) – joescii Mar 11 '14 at 14:00

Scala compile warning with pattern matching

2 Answers2