> the type system ensured that, unless the capability had been revoked, illegal requests would not type-check at compile time

I don't think I can imagine what you mean here. How would my compilation success depend on whether an Admin has given me the right to read or write to a certain path?

First, I am talking about the capability based HTTP APIs I have worked on here. Not some imagined operating system. I can see if I get around to your question in the other sub thread.

But the point is, you would not grant access to a “path” or some such. You gain access to a collection of actions, described by types. It can be simple “get/set property” or more complicated actions, maybe involving capabilities to several things at once.

To mint such an access token the “admin” will have say how the actions are performed, and then pass the token to you. The client code then will have a typed set of actions it can perform using the token.

This does not mean that we compile in the specific rights of each user or anything, it only means that we must ensure a valid flow of rights in order for the whole program to compile. You can get a capability in two ways: you mint a capability for something you already can do (you are the admin and own the data base, say), or you receive it (via an API call, say).

There is definitely a knack to design the system with this kind of flow. But once in place it feels very natural, you get the right to do something just in time for your need to do it. Very much principle of least privilege taken to an extreme.