For anybody curious, I think he's hinting at modern effect systems, on which there's lots of research these days. He advised for instance Lukas Rytz PhD thesis

For anybody curious, I think he's hinting at modern effect systems, on which there's lots of research these days. He advised for instance Lukas Rytz PhD thesis at EPFL (and Lukas Rytz is still working on Scala, now at Lightbend).