Scala's type system provides generics in two ways—type members and type parameters—and most advanced Scala developers will use both in their code at different times.

But how do we choose between them? Usually one or the other will feel "right" for a particular use case, and, if not, we can usually refactor our code and take the alternative approach. But could we learn to be better at making the "right" choice first time?

I'll explore the most important considerations when choosing between them, their relationship with existential and universal quantification (I'll also explain what this means!), and look at whether they are completely interchangeable in all cases, or whether certain scenarios force us to take one path or the other.

Jon (@propensive) has been having fun riding the bleeding edge of Scala for over a decade, and he's not done yet. While he's not travelling the world attending Scala conferences, or organizing his own (Scala World), Jon spends his time working on the open-source Rapture libraries. He is principal engineer at The Scala Center.