Question

I’m working through the O’Reilly Programming Scala book, and have run into a stumbling block with this code sample:

/* matching on sequences */
val willWork = List(1, 3, 23, 90 );
val willNotWork = List( 4, 18, 52 );
val empty = List();

for( l <- List(willWork, willNotWork, empty ))
{
    l match
{
  case List( _, 3, _, _ ) => println( "Four elements, with the second being '3'." );
  case List( _* ) => println( "Any other list with zero or more elements" );
  case _ => println( "Uh, oh!" );
}
}     

According to the text, the List( _* ) should match any List with zero or more elements, but when I execute this, the List(4,18,52) does not match, and falls into the case _ section (or, if that’s removed, throws a MatchError).

Any idea why this isn’t matching? Has there been a language change since the book was published, or do I just have one of those “typos that you can’t see yourself” things going on?

Answer 1

What version of Scala do you use?

In Scala 2.8.1.final, it will complain the last case is unreachable.

scala> val willWork = List(1, 3, 23, 90 );
willWork: List[Int] = List(1, 3, 23, 90)

scala> val willNotWork = List( 4, 18, 52 );
willNotWork: List[Int] = List(4, 18, 52)

scala> val empty = List();
empty: List[Nothing] = List()

scala> 

scala> for( l <- List(willWork, willNotWork, empty ))
     | {
     |     l match
     | {
     |   case List( _, 3, _, _ ) => println( "Four elements, with the second being '3'." );
     |   case List( _* ) => println( "Any other list with zero or more elements" );
     |   case _ => println( "Uh, oh!" );
     | }
     | }     
<console>:15: error: unreachable code
         case _ => println( "Uh, oh!" );
                          ^

scala> 

And it works fine to match the empty list.

scala> val willWork = List(1, 3, 23, 90 );
willWork: List[Int] = List(1, 3, 23, 90)

scala> val willNotWork = List( 4, 18, 52 );
willNotWork: List[Int] = List(4, 18, 52)

scala> val empty = List();
empty: List[Nothing] = List()

scala> 

scala> for( l <- List(willWork, willNotWork, empty ))
     | {
     |     l match
     | {
     |   case List( _, 3, _, _ ) => println( "Four elements, with the second being '3'." );
     |   case List( _* ) => println( "Any other list with zero or more elements" );
     | }
     | }
Four elements, with the second being '3'.
Any other list with zero or more elements
Any other list with zero or more elements

scala>