MatchError when match receives an IndexedSeq but not a LinearSeq

Question

Is there a reason that `match` written against `Seq` would work differently on `IndexedSeq` types than the way it does on `LinearSeq` types? To me it seems like the code below should do the exact same thing regardless of the input types. Of course it doesn't or I wouldn't be asking. ``` import collection.immutable.LinearSeq object vectorMatch { def main(args: Array[String]) { doIt(Seq(1,2,3,4,7), Seq(1,4,6,9)) doIt(List(1,2,3,4,7), List(1,4,6,9)) doIt(LinearSeq(1,2,3,4,7), LinearSeq(1,4,6,9)) doIt(IndexedSeq(1,2,3,4,7), IndexedSeq(1,4,6,9)) doIt(Vector(1,2,3,4,7), Vector(1,4,6,9)) } def doIt(a: Seq[Long], b: Seq[Long]) { try { println("OK! " + m(a, b)) } catch { case ex: Exception => println("m(%s, %s) failed with %s".format(a, b, ex)) } } @annotation.tailrec def m(a: Seq[Long], b: Seq[Long]): Seq[Long] = { a match { case Nil => b case firstA :: moreA => b match { case Nil => a case firstB :: moreB if (firstB < firstA) => m(moreA, b) case firstB :: moreB if (firstB > firstA) => m(a, m…

Accepted Answer

You can't use `::` for anything other than `List`. The `Vector` is failing to match because `::` is a case class that extends `List`, so its `unapply` method does not work for `Vector`.

```
val a :: b = List(1,2,3)    // fine
val a :: b = Vector(1,2,3)  // error
```

But you can define your own extractor that works for all sequences:

```
object +: {
  def unapply[T](s: Seq[T]) =
    s.headOption.map(head => (head, s.tail))
}
```

So you can do:

```
val a +: b = List(1,2,3)   // fine
val a +: b = Vector(1,2,3) // fine
```

MatchError when match receives an IndexedSeq but not a LinearSeq

Solution

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