[llemiengre]

This is no longer the case, in Scala 2.8 they were replaced with persistent collections based on the work by Phil Bagwell, afaik they are faster than the old implementation. Didn't the old implementation have issues when you started sharing these maps between threads?

[seanmcdirmid]

I haven't used Scala since 2006, so I'm not sure; I always avoided immutable collections (beyond lists sometimes) in favor of mutable ones given the nature of my work then (IDE development). I suspected they weren't thread safe, but Martin was confident at the time.