Question

Thanks for the excellent example, I tried it and it works as I expected. Nice to see someone understood the nature of the problem. However, I think I should have tagged the problem with Lift as I’m using the Lift framework and that is where this problem is (still) occurring (although I still think it might be related to extraction in scala). Since I don’t want to reproduce the entire Lift setup here as it will be too much code, I’m going to hope someone familiar with Lift can understand what I’m doing here. I’ve removed more variables so it might be easier (for some) to see the problem:

lazy val dispatch: LiftRules.DispatchPF = {
  // Explicitly setting guard to false to trigger the scenario
  case req: Req if false => () => println("shouldn't match"); Empty
  // This should match since previous case will never match
  case Req(_, _, _) => () => println("should match"); Empty
  // This is actually called...
  case _ => () => println("shouldn't reach here"); Empty
}

As before, if I comment out the first case the second case is matched as expected.

For those interested, a simple workaround is:

lazy val dispatch: LiftRules.DispatchPF = {
  case req: Req => {
    if (false) { // Obviously you put something more useful than false here...
      () => println("shouldn't match"); Empty
    } else req match {
      // This matches
      case Req(_, _, _) => () => println("should match"); Empty
      // This is now never called
      case other => () => println("shouldn't reach here"); Empty
    }
  }
}

ORIGINAL POST

I’m new to scala, so I may be doing something wrong here, but I have a pattern matching expression that seems to be skipped over. Here’s the code:

lazy val dispatch: LiftRules.DispatchPF = {
   // Explicitly setting guard to false to trigger the scenario
   case req: Req if false => () => Full(...)
   // This should match since previous case will never match
   case Req("api" :: "test" :: Nil, suffix, GetRequest) => () => Full(...)
   // This is actually called...
   case _ => () => println("not sure what's going on"); Empty
}

If I take out the first case expression, everything works as expected. I’m tempted to think this is a bug (https://issues.scala-lang.org/browse/SI-2337), but does anyone know of a workaround?

Answer 1

This is indeed the bug you are referencing in the Scala bug tracker. Req is a non-case class with a companion extractor methods, so the bug manifests itself here. The workaround you introduced seems fine.

For those interested, here is a sample case where the bug manifests itself:

class Sample(val a: String)

object Sample {
    def apply(a: String) = new Sample(a)
    def unapply(s: Sample) = Option(s.a)
}

val s = new Sample("a")

val r = s match {
    case n: Sample if false => "Wrong 1: " + n
    case Sample(_) => "Yay" 
    case n => "Wrong 2: " + n
}

println("Found " + r)
assert(r == "Yay")