I actually can't tell, but it has not yet been significant. It takes quite a bit of time to really get a library like this started. So far we've mostly dealt with people who are using xtensor from C++ or bind it to Python.

We've mainly gone through RCpp for the R language, and that has been working great. I don't know about changes in R 3.5 or ALTREP. Is there something we should know/change for it?