parameters to traits in collection library.

[G J]

I embarrassed to say that I have been using Scala for some time, but I find one cornerstone hard to understand - among others. It's the type parameters used in much of the collection library. Is there a pointer that explains this? Specifically:

    trait IndexedSeq[+A]                                                                                        (1)

           extends Seq[A] with GenericTraversableTemplate[A, IndexedSeq]            (2)

                with IndexedSeqLike[A, IndexedSeq[A]]                                               (3)

• On (2): why is it IndexedSeq and not IndexedSeq[A]; I mean, what is IndexedSeq on its own anyway.
• On (3) we are defining IndexedSeq[+A] in terms of IndexedSeq[A].  I don't understand the recursion here.
• On (3) that type  parameter is supposed to be the underlying representation for IndexedSeq[A]. Declaring it the way shown appears to be circular.
  

Thanks.

Best Regards.

[Axel Poigné]

Some of your questions may be answered by

http://docs.scala-lang.org/overviews/core/architecture-of-scala-collections.html

[Sonnenschein]

• On (2): why is it IndexedSeq and not IndexedSeq[A]; I mean, what is IndexedSeq on its own anyway.

 See http://stackoverflow.com/questions/6246719/what-is-a-higher-kinded-type-in-scala.

• On (3) that type  parameter is supposed to be the underlying representation for IndexedSeq[A]. Declaring it the way shown appears to be circular.
  

Circular dependencies would be detected by the compiler as error.