Scala inline unapply extractor

Question

The following code:

object Test {
  @inline def unapply(i: Int): Option[String] =
    i match {
      case 1 => Some("Got 1")
      case 2 => Some("Got 2")
      case 3 => throw new Exception("Should not test 3")
      case _ => None
  }
  def test(i: Int) = i match {
    case Test(k) => k
    case 4 => "Another 4"
    case _ => ""
  }
}
Test.test(3)

results in the following error:

...
at Test$.unapply(<console>:13)
at Test$.test(<console>:17)
...

Note that it's clear where the error comes from. However, the error shows that the method unapply is not inlined as I wanted to.
How can I inline this unapply method? This is for performance reason as well as code reuse.

I'm not sure, but could Scala maintain the original stack-trace even though the method is inlined? — marstran, Nov 29 '16 at 13:28
@marstran I checked the disassembly and I can confirm that the method is not inlined. Indeed the JVM allows to produce debug code on inline functions. — Mikaël Mayer, Nov 29 '16 at 13:46
Did you have a chance to check exception errors list and check scala sources??? — Pavel, Nov 29 '16 at 13:52
What do you mean by "checking exception errors list" ? The one usually at the bottom of the disassembled file? There isn't any as far as I see. — Mikaël Mayer, Nov 29 '16 at 13:58
@MikaëlMayer why do you want to inline ? what performance reasons. What is the performance gain you are expecting by doing this ? — Nagarjuna Pamu, Nov 29 '16 at 14:00
Remove one layer of stack call while being able to refactor the code. — Mikaël Mayer, Nov 29 '16 at 14:05

score 3 · Accepted Answer · edited May 23 '17 at 11:45

@inline only requests that the compiler try to inline a method, but it doesn't have to, and in some cases, it can't. I don't think there is a spec for exactly what can and cannot be inlined by the compiler (though I'd love to see one if there is), but I'd bet the compiler simply won't in this case, no matter what you do.

Generally, you would only want to inline something that is a constant or small and unlikely to change, anyway. Otherwise, many uses of Test.unapply will cause the compiled code to increase in size quickly, and changes to the inlined method will proliferate across everything that references it.

This answer from @RexKerr says enough.

Scala inline unapply extractor

1 Answers1