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)
