Their new repl is called cling: https://github.com/root-project/cling (They had an old one called C-INT or something like that).

One of the biggest problem with these repl for complied languages is that they panic as soon as you rerun a command. `cling` integrates with Jupyter notebook too but as soon as you rerun a declaration of a variable, everything collapse and you have to reset the kernel. Maybe I didn’t dig deep enough and there is a way around this but if not, this is quite painful to deal with and a deal breaker.

So repeating something like 'int i = 42;' causes problems?

Huh, I didn't know about that tool. Looking through the docs this looks impressive, especially since it can do C++ too.