My goal is to create a function that takes varargs of 2 or more objects when initializing a PriorityQueue containing said objects.
The relevant code is:
case class Topic(topic: String, usageFrequency: Long = 1)
object FreqOrdering extends Ordering[Topic] {
def compare(a: Topic, b:Topic) = -(a.usageFrequency compare b.usageFrequency)}
def initPriQu(a : Topic, b: Topic, c: Topic*): PriorityQueue[Topic] = {
return PriorityQueue(a,b,c)(FreqOrdering)}
Error in sbt (Scala 2):
[error] found : TopicTrenderInit.FreqOrdering.type
[error] required: scala.math.Ordering[Equals]
[error] Note: TopicTrenderInit.Topic <: Equals (and TopicTrenderInit.FreqOrdering.type <: scala.math.Ordering[TopicTrenderInit.Topic]), but trait Ordering is invariant in type T.
[error] You may wish to investigate a wildcard type such as_ <: Equals. (SLS 3.2.10)
[error] return PriorityQueue(a,b,c)(FreqOrdering)
[error] ^
[error] /home/aaron-laptop/Documents/Scala/topic_trender100/src/main/scala/main.scala:48:25: type mismatch;
[error] found : scala.collection.mutable.PriorityQueue[Equals]
[error] required: scala.collection.mutable.PriorityQueue[TopicTrenderInit.Topic]
[error] Note: Equals >: TopicTrenderInit.Topic, but class PriorityQueue is invariant in type A.
[error] You may wish to investigate a wildcard type such as_ >: TopicTrenderInit.Topic. (SLS 3.2.10)
[error] return PriorityQueue(a,b,c)(FreqOrdering)
When there is no '*' indicating a vararg everything works, no errors. I think what confuses me the worst is the required: scala.math.Ordering[Equals] error I'm seeing. I also read an article on pattern matching, but I feel i'll have to read more on it to understand implementation. What's going on here?
Thanks.
The problem is the way you are building the PriorityQueue. You are passing it two values of type Topic and one of type Seq[Topic] so the result is PriorityQueue[Any].
This should work:
def initPriQu(a : Topic, b: Topic, c: Topic*): mutable.PriorityQueue[Topic] =
mutable.PriorityQueue(Seq(a, b) ++ c:_*)(FreqOrdering)
Also, don't use return.