I think there could be another category made on this forum, dedicated to discussing specific clauses of the book for learners who are getting stuck with some parts of the book. The book does a really great job in being self-contained and impeccably organised. However the authors still thought it necessary linking to a forum to discuss the book, which is currently hosted on Artima website and lies dormant. I suggest opening the “Programming in Scala Book Forum” in this scala-users discourse portal. And perhaps its link could be officially included in future versions of the book.