Yes, we additionally have functions builtin to the runtime that let you evaluate a term before you evaluate it to make sure it doesn't call any "forbidden" functions.

So in our cloud runtime, we blacklist EVERY IO function, then we can in our cloud runtime give you back the ability to do, for example, http requests, but not any other network IO. We won't let you open arbitrary files, but we'll provide ephemeral / persistent block storage through some other runtime ability.

This could also be used to do something like blacklist functions with known security vulnerabilities to catch people that aren't applying their patches!