Mixing typed matches with sequence matcher gives out strange behaviour in Scala

Question

I'm trying to figure out what's going on on this piece of code, trying to figure out if there is something I don't understand or if it is a compiler bug or unintuitive spec, let's define these two almost identical functions:

def typeErause1(a: Any) = a match {
    case x: List[String] => "stringlists"
    case _ => "uh?"
}
def typeErause2(a: Any) = a match {
    case List(_, _) => "2lists"
    case x: List[String] => "stringlists"
    case _ => "uh?"
}

now if I call typeErause1(List(2,5,6)) I get "stringlists" because even if it is actually List[Int] with type erasure it is not able to tell the difference. But strangely if I call typeErause2(List(2,5,6)) I get "uh?" and I don't understand why it is not matching List[String] like it did before. If I use List[_] instead on the second function it is able to match it correctly which makes me think this is a bug in scalac.

I'm using Scala 2.9.1

It's a bug in the matcher -- there are plenty tickets about it, and it should work starting with Scala 2.10.x. — Daniel C. Sobral, Apr 08 '12 at 05:13
do you have a link to the ticket so I can choose you as the answer? — ilcavero, Apr 08 '12 at 13:14
As I said, there are plenty tickets about the matcher, and I'm really not particularly interested in hunting for the one that covers this particular case. — Daniel C. Sobral, Apr 08 '12 at 23:52
well thanks for your opinion but I was looking for a sustained reason and "it's a bug in the matcher because there are plenty of tickets about the matcher" is not one. — ilcavero, Apr 09 '12 at 02:13
I'm not "opining" here, and you have as much access to the ticket list as I am. Stack Overflow is not "let others do the look up for me". — Daniel C. Sobral, Apr 09 '12 at 12:21
@ilcavero - Daniel didn't answer; he left a comment. You're welcome to wait for someone to answer with supporting links, or look it up and answer yourself. Rudeness, however, [is not welcome](http://stackoverflow.com/faq#etiquette). — leedm777, Apr 09 '12 at 14:15

score 1 · Accepted Answer · edited May 23 '17 at 12:03

It's a bug in the matcher ;) The pattern matcher is being (has been?) rewritten for 2.10

I just checked with the latest nightly and your code works as expected:

Welcome to Scala version 2.10.0-20120426-131046-b1aaf74775 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_31).
Type in expressions to have them evaluated.
Type :help for more information.

scala> def typeErause1(a: Any) = a match {
     |     case x: List[String] => "stringlists"
     |     case _ => "uh?"
     | }
warning: there were 2 unchecked warnings; re-run with -unchecked for details
typeErause1: (a: Any)String

scala> def typeErause2(a: Any) = a match {
     |     case List(_, _) => "2lists"
     |     case x: List[String] => "stringlists"
     |     case _ => "uh?"
     | }
warning: there were 3 unchecked warnings; re-run with -unchecked for details
typeErause2: (a: Any)String

scala> typeErause1(List(2,5,6))
res0: String = stringlists

scala> typeErause2(List(2,5,6)) 
res1: String = stringlists

Mixing typed matches with sequence matcher gives out strange behaviour in Scala

1 Answers1