scala match on iteration, weird compiler error - scala

Question

This small piece of scala code when compiled
it gives a strange error.
import scala.collection.mutable.ListBuffer
object main {
def main(args: Array[String]) {
val rs = new ListBuffer[String]()
val ns = scala.collection.mutable.Map[String, String]()
"A very long string".split("\\ ") foreach { word =>
word match {
case x if x.length() > 7 => ns += x -> "="
case _ => rs += word
}
}
}
}
Gives the following error:
test.scala:11: error: type arguments [String,Iterable[java.io.Serializable] with PartialFunction[String with Int,String] with scala.collection.generic.Subtractable[String,Iterable[java.io.Serializable] with PartialFunction[String with Int,String] with scala.collection.generic.Subtractable[String,Equals]]{def seq: Iterable[java.io.Serializable] with PartialFunction[String with Int,String]}] do not conform to trait Subtractable's type parameter bounds [A,+Repr <: scala.collection.generic.Subtractable[A,Repr]]
"A very long string".split("\\ ") foreach { word =>
^
one error found
Any hints?

Answer 1

This happens due to the fact that your pattern match returns a lengthy compound type:
[String,Iterable[java.io.Serializable] with PartialFunction[String with Int,String] with scala.collection.generic.Subtractable[String,Iterable[java.io.Serializable] with PartialFunction[String with Int,String] with scala.collection.generic.Subtractable[String,Equals]]{def seq: Iterable[java.io.Serializable] with PartialFunction[String with Int,String]}]
Where one of the types in the linearization is Subtractable[String, Iterable[Serializable]] which your type doesn't conform to it's type constraint:
trait Subtractable[A, +Repr <: Subtractable[A, Repr]
To fix this, help the compiler out with type annotation on foreach:
"A very long string"
.split("\\")
.foreach[Unit] {
case x if x.length() > 7 => ns += x -> "="
case word => rs += word
}
As a side note, perhaps you can find the following implementation helpful:
val (largerThanSeven, smallerThanSeven) = "A very long string"
.split("\\")
.partition(_.length > 7)
val largerThanSevenMap = largerThanSeven.map(_ -> "=").toMap

Answer 2

The problem is here:
ns += x -> "="
I guess the intention is to put the key x and the value "=" into the mutable map.
This will compile if changed to
ns(x) = "="
The problem is that there are implicits involved into conversions with -> and the types are inferred weirdly into the monstrous string in the error message.