PL designer here with extensive experience writing compilers in C#, Java, and Scala. Pattern matching is nice but OO will get you most of the way and filling the matching gap is simple.
The expression problem is much safer solved using virtual classes (using traits and type members) in scala than with case matching. There are so many OO solutions to this problem it's not funny; I think I had a small argument with Wadler over this back in 2001 or 2.
As for persistent collection types, observable collections with undoable operations are just as good, if not better because they can still support mutability while persistent collections cannot.
The expression problem is much safer solved using virtual classes (using traits and type members) in scala than with case matching. There are so many OO solutions to this problem it's not funny; I think I had a small argument with Wadler over this back in 2001 or 2.
As for persistent collection types, observable collections with undoable operations are just as good, if not better because they can still support mutability while persistent collections cannot.