Inheritance and type parameters of Traversable
- by Jesper
I'm studying the source code of the Scala 2.8 collection classes. I have questions about the hierarchy of scala.collection.Traversable. Look at the following declarations:
package scala.collection
trait Traversable[+A]
extends TraversableLike[A, Traversable[A]]
with GenericTraversableTemplate[A, Traversable]
trait TraversableLike[+A, +Repr]
extends HasNewBuilder[A, Repr]
with TraversableOnce[A]
package scala.collection.generic
trait HasNewBuilder[+A, +Repr]
trait GenericTraversableTemplate[+A, +CC[X] <: Traversable[X]]
extends HasNewBuilder[A, CC[A] @uncheckedVariance]
Question: Why does Traversable extend GenericTraversableTemplate with type parameters [A, Traversable] - why not [A, Traversable[A]]? I tried some experimenting with a small program with the same structure and got a strange error message when I tried to change it to Traversable[A]:
error: Traversable[A] takes no type parameters, expected: one
I guess that the use of the @uncheckedVariance annotation in GenericTraversableTemplate also has to do with this? (That seems like a kind of potentially unsafe hack to force things to work...).
Question: When you look at the hierarchy, you see that Traversable inherits HasNewBuilder twice (once via TraversableLike and once via GenericTraversableTemplate), but with slightly different type parameters. How does this work exactly? Why don't the different type parameters cause an error?
