I want to say that BeOS did this back in the day. I don’t think it was specifically a FUSE mount, but rather that the contacts application read vcards (or somet

I want to say that BeOS did this back in the day. I don’t think it was specifically a FUSE mount, but rather that the contacts application read vcards (or something similar) stored as plain files in the file system. Then the Contacts app just needed to be pointed to the proper folder and it would work. I definitely remember the email application working that way, but think contacts worked that way too. It was quite a clever way of interacting with your system. Everything was still a file, but your client applications were a bit more clever in how they interacted with those individual files. It was very close to the file system as a DB concept that was popular at the time, but far more practical.