[lmm]

The Scala collections library has a fair bit of second-system effect and plain bad design. It's being rewritten with simplification as an explicit goal (even at the cost of sacrificing some expressiveness) for the next major version of Scala. In the meantime there are a couple of alternative libraries, and fundamentally the standard one does work. (If you need to implement a custom collection the recommendation is to only implement an interface - or maybe even just a suitable typeclass - and not try and reuse any implementation traits/base classes from the standard library).

Which is to say, I don't blame you, but FWIW collections are a known-bad part of the language and exemplify a lot of what the community wants to move away from. I find Scala is great for being able to read the implementation of things, particularly in terms of how much the frameworks tend to be written in plain old Scala code rather than magic annotations or what have you (e.g. in Spray/akka-http, all the directives you use to define your web routes are just ordinary Scala functions and you can click through and read their definitions).