Eval considered

Using eval is almost always a bad idea. It's a crutch for bad designs, it opens giant security holes in your code, and in practice it's almost never needed to get actual stuff done.

Actually, mutable state is useful in concurrent programs too. If you have 1 speaker thread and N listener threads, then mutable global shared state is the cleanest and most efficient way to go. To avoid that, you'd have to send messages to all N threads, which would require the speaker thread to keep track of its listeners... fuck, give me mutable shared state any day.