I have a recursive function count
in Scala 2.9.2 that looks like this
def count(traces: Seq[(Char, Char)], acc: (TP, TN, FP, FN)): (TP, TN, FP, FN) = {
val (tp, tn, fp, fn) = acc
traces match {
case Nil => acc
case ('(', '(')::rest => count(rest, (tp + 1, tn, fp, fn))
case (')', ')')::rest => count(rest, (tp + 1, tn, fp, fn))
case ('(', ')')::rest => count(rest, (tp, tn + 1, fp, fn))
// ... exhaustive set of cases ...
}
}
On input Seq(('(', '('))
the function throws the following MatchError
:
scala.MatchError: Vector(((,()) (of class scala.collection.immutable.Vector)
I investigated this by playing around with the code in the Scala console.
scala> val t = Seq(('a', 'b'), ('b', 'c'))
t: Seq[(Char, Char)] = List((a,b), (b,c))
scala> t match { case Nil => "h"; case ('a', 'b')::rest => rest }
res6: java.lang.Object = List((b,c))
scala> t1 match { case Nil => "h"; case ('a', 'b')::rest => rest }
scala.MatchError: List((b,c)) (of class scala.collection.immutable.$colon$colon)
It seems as if matching ('a', 'b')::rest
(the second line) doesn't return an object of the correct type, since the Seq[(Char, Char)]
is suddenly of type java.lang.Object
which Scala then doesn't know how to match on.
What explains this behavior?