stack overflow on overriding lazy val in scala - scala

Question

I have trimmed my code down to the following. I am confused why I am getting a stack overflow between the two filter methods (one in my trait and one in my superclass)
object TestingOutTraits {
val TestHandler = new Object with MySuper with MyTrait {
override lazy val createdFilter = {
"second part"
}
}
def main(args: Array[String]) = {
val result : String = TestHandler.start()
System.out.println("result="+result)
}
}
trait MySuper {
protected def filter: String = {
"first part to->"
}
def start() = {
filter
}
}
trait MyTrait { self: MySuper =>
lazy val createdFilter = {
"override this"
}
protected override def filter: String = {
self.filter + createdFilter
}
}
This is scala 2.9. Any ideas what is going on here?
EDIT:
The stack trace makes no sense on how it jumps back and forth too(I should have included it in original post)...
at MyTrait$class.filter(TestingOutTraits.scala:34)
at TestingOutTraits$$anon$1.filter(TestingOutTraits.scala:4)
at MyTrait$class.filter(TestingOutTraits.scala:34)
at TestingOutTraits$$anon$1.filter(TestingOutTraits.scala:4)
thanks,
Dean

Answer 1

The call self.filter in MyTrait.filter invokes itself, leading to infinite recursion that blows the stack.
Instead, have MyTrait extend MySuper, and use super.filter:
trait MyTrait extends MySuper {
lazy val createdFilter = {
"override this"
}
protected override def filter: String = {
super.filter + createdFilter
}
}

Answer 2

Alternatively,
trait MySuper extends Filtered {
protected def filter: String = {
"first part to->"
}
def start() = {
filter
}
}
trait Filtered {
protected def filter: String
}
trait MyTrait extends Filtered {
lazy val createdFilter = {
"override this"
}
protected abstract override def filter: String = {
super.filter + createdFilter
}
}
then
val nope = new MyTrait { } // correctly DNC
and the OP
val TestHandler = new MySuper with MyTrait {
override lazy val createdFilter = {
"second part"
}
}
http://www.artima.com/pins1ed/traits.html#12.5

Answer 3

Well, I know why the infinite recursion though this seems like a linearization bug in scala and I am not sure how to work around it yet either :(.
hmmmm, so it turns out the compiler is sticking a filter method in my new Object for some reason. I found this out with the print:mixin on scalac like so
$ scalac -Xprint:mixin TestingOutTraits.scala
[[syntax trees at end of mixin]]// Scala source: TestingOutTraits.scala
package <empty> {
final object TestingOutTraits extends java.lang.Object with ScalaObject {
private[this] val TestHandler: MySuper = _;
<stable> <accessor> def TestHandler(): MySuper = TestingOutTraits.this.TestHandler;
def main(args: Array[java.lang.String]): Unit = {
val result: java.lang.String = TestingOutTraits.this.TestHandler().start();
java.this.lang.System.out.println("result=".+(result))
};
def this(): object TestingOutTraits = {
TestingOutTraits.super.this();
TestingOutTraits.this.TestHandler = {
new anonymous class TestingOutTraits$$anon$1()
};
()
}
};
abstract trait MySuper extends java.lang.Object with ScalaObject {
def filter(): java.lang.String;
def start(): java.lang.String
};
abstract trait MyTrait extends java.lang.Object with ScalaObject { self: MyTrait =>
def createdFilter(): java.lang.String;
override def filter(): java.lang.String
};
abstract trait MySuper$class extends {
def filter($this: MySuper): java.lang.String = "first part to->";
def start($this: MySuper): java.lang.String = $this.filter();
def /*MySuper$class*/$init$($this: MySuper): Unit = {
()
}
};
abstract trait MyTrait$class extends { self: MyTrait =>
def createdFilter($this: MyTrait): java.lang.String = "override this";
override def filter($this: MyTrait): java.lang.String = $this.$asInstanceOf[MySuper]().filter().+($this.createdFilter());
def /*MyTrait$class*/$init$($this: MyTrait): Unit = {
()
}
};
final class TestingOutTraits$$anon$1 extends java.lang.Object with MySuper with MyTrait {
override def filter(): java.lang.String = MyTrait$class.filter(TestingOutTraits$$anon$1.this);
def start(): java.lang.String = MySuper$class.start(TestingOutTraits$$anon$1.this);
override def createdFilter(): java.lang.String = "second part";
def this(): anonymous class TestingOutTraits$$anon$1 = {
TestingOutTraits$$anon$1.super.this();
MySuper$class./*MySuper$class*/$init$(TestingOutTraits$$anon$1.this);
MyTrait$class./*MyTrait$class*/$init$(TestingOutTraits$$anon$1.this);
()
}
}