My understanding is that Dotty is the next generation of the Scala compiler. See the Scala roadmap for the details: http://www.scala-lang.org/news/roadmap-next. Basically, it's the culmination of research showing that the language's feature set can actually be expressed in a smaller number of primitives than previously thought. A lot of features that are distinct today will become syntactic sugar for more general underlying representations. I think the expectation is that this will actually make the compiler more agile going forward,and let the maintainers cut out some of its gnarliest corners.