Your macro has problems with variable capture and multiple evaluation (see chapters 9-10 of On Lisp). Here's a version that should work properly:
(mac bias args
(w/uniq r
(withs (ws (map car (pair args))
xs (map cadr (pair args))
us (map [uniq] ws))
`(with ,(mappend list us ws)
(let ,r (rand (+ ,@us))
(if ,@(mappend
(fn (u x) `((< (-- ,r ,u) 0) ,x))
us xs)))))))
IMO, though, the use of a macro here is a premature optimization. I think you should try to get a function working first, and then wrap a macro around it if you know that's what you need. See my comment http://arclanguage.org/item?id=7760 for an example of such a wrapper macro (in CL, but the Arc is similar).
