This one is very similar to the definition of rand-choice. (Why does the mappend expression work? Aren't the values of the biases unavailable during macro-expansion?)
This version still needs some massaging but it works with non-constant biases:
(mac bias args
(let bs (map car (pair args))
`(let r (rand (+ ,@bs))
(if ,@(let i 0
(rev (accum a
(each c (map cadr (pair args))
(a `(< r (+ ,@(cut bs 0 (++ i)))))
(a c)))))))))
arc> (with (a 1 b 2 c 3) (bias a 'red b 'white c 'blue))
white
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).
I still suggest you take a look at how I do it http://arclanguage.com/item?id=7765 , which (1) avoids multiple evaluation, and (2) avoids variable capture.
(1) is the hard part here, which is why I had to use a list.
