[zeveb]

As an example, Lisp has an amazingly full-featured condition system. It enables all sorts of stuff like detecting an error condition, changing arguments, rerunning code, replacing code while it's running &c. If it were plumbed all the way down into the OS, I could call lispy_mmap with the wrong arguments, detect that they're wrong, fix them and then return from lispy_mmap as though nothing had happened.

Instead, in a Lisp on Unix, I'll call mmap and either have to manually check for an error or use a wrapper which does that for me. If there is an error, I'll have to re-call mmap.

It's not terrible, and it's still a lot better than using e.g. C — but it's not as good as it could be in a Lisp-all-the-way-down system.