Scala collections API is rather nice.
The way that both mutable and immutable types follow the same tree structure makes it very elegant.
I was just wandering if there is anything out there that implements Scala-like collections.
Oh, and don’t say C5 because it does not have truly immutable collections.
.NET just shipped their first immutable collections, which I suggest you try out.
In particular, these with .NET 4.5 offer the shared read only interfaces that work on both mutable and immutable collections that it sounds like you’re looking for, while offering efficient copy-on-write implementations of collections.