What about immutable?

[johnynek]

a specialized immutable list might be of use. Likewise Vector and HashTrieMap/Set. Especially with concurrency, sometimes the immutable versions are better (when you need to provide snapshotting without copying).

[rklaehn]

Once it is finally settled where basic typeclasses like Eq, Monoid, Order, Hash, Show live (spire.algebra, algebra, cats, whatever), I could port over the data structure underlying s.c.i.HashTrie/HashSet. Deep under all the scala collections methods it is actually quite beautiful. I implemented tree/tree operations for it in scala 2.11.

I also started working on some immutable, specialized collections in https://github.com/rklaehn/abc . But I have to pull in Show from cats and provide my own Hash, which is less than optimal.