It makes sense that they wouldn't do that at the vm level. Your code even makes sense, though I thought "let" only assigned one variable.
I'm still not quite able to read arc fluently, so any explanations of the subtle that I likely missed will always be appreciated. Come to think of it, any explanations of any code would be nice, as the thoughts and reasons behind code don't always come out in the source itself. And I also like learning new things :)
I'm using pattern matching. Although Arc doesn't actually have pattern-matching built in, someone wrote a pattern matching library a long time ago using macros: http://arclanguage.com/item?id=2556http://arclanguage.org/item?id=1825 . The modern evolution uses something like p-m:def to define a pattern-matching function, p-m:fn to create an anonymous pattern-matching function, etc.
('request pid tag param)
The pattern above means "match a 4-element list, whose first element is the symbol 'request, and which has 3 more elements that we'll call pid, tag, and param".
(let (state . response) (fun state param)
This is a destructuring. It simply means that (fun state param) should return a cons cell, with the 'car of the cell being placed in state and the 'cdr being placed in response. So we expect fun to return something like (cons state response)
(==> pid (list 'response tag response))
Note the use of 'tag here. We expect that 'tag would be a 'gensym'ed symbol, and is used in pattern matching so that the client can receive the message it's looking for.
(gen-server fun state))
Plain recursion.
; hot code swapping!!
('upgrade new-fun)
(gen-server new-fun state)
