Hmm, a different way of looking at the issue just occurred to me: pushing atomic inside of expansions of = may make Arc plus News shorter but Arc plus my program longer.
If ++ didn't do locking, then in places where you were using x from multiple threads you'd need to say (atomic (++ x)). Pushing atomic inside ++ makes this shorter because now you can just say (++ x). But pushing atomic into expansions of = also means that locking occurs at other times, and so I can't use it in places where I'm doing things like throw and readfile that break with locking, which makes my program longer.
Which leads to a fascinating idea, if we get a large enough body of open source Arc code that we can start optimizing for code size globally... :)
That would increase the conceptual load of programming in Arc a lot. It would make people have to think about the expansions of operators like ++ to know when to wrap things in atomic and when not to. You need to be able to treat built-in operators like that as black boxes. Once you start thinking about macroexpansions, it's as if you had to write them.
Hmm, well, I can only speak with any knowledge about my own conceptual load... I expect with your background (professor, Lisp book author, tutorial writer, mentor, etc.) you have a much better idea of what other programmers would find easy or difficult.
I know that some things should be atomic, such as accessing shared mutable data structures, and some things that I need to avoid being atomic, such as doing I/O.
I find Arc's making some operations atomic for me doesn't help me all that much, because without knowing the details of the macroexpansion, I don't know if everything that needs to be atomic has been made so. And I find it unhelpful in other cases, when I need an operation to not be atomic, and so I need to look at the macroexpansion of = to find out if that particular expansion is doing something that I need it to not do, or if it's ok.
On the other hand, I have no alternative to offer yet ^_^. I surmise that if I factored Arc + News + my code, perhaps I might come up with a useful suggestion to offer, and if I do, I'll certainly post about it!
