Comment by harpocrates

This is a fun data structure to implement in Haskell, and I've always been curious about how one would do it in C++, largely due to the fact that

    data FingerTree a = Empty
                      | Single a
                      | Deep (Digit a) (FingerTree (Node a)) (Digit a)

Has polymorphic recursion in the last case. What would be the C++ approach for dealing with this sort of thing? Pass in a compile time integer to represent the level of nesting?