prob cond

prob-cond.clj

(defmacro prob-cond [& clauses]
  (let [valsym (gensym "val_")
        test-vals (reductions + (take-nth 2 clauses))]
    `(let [~valsym (* ~(last test-vals) (rand))]
       (cond
         ~@(mapcat (fn [tv clause] `[(<= ~valsym ~tv) ~clause])
             test-vals
             (take-nth 2 (rest clauses)))))))

(prob-cond
  2 :hi
  2 :there
  3 :marty)

;; expands to:

(let* [val_1329 (* 7 (rand))]
  (cond
    (<= val_1329 2) :hi
    (<= val_1329 4) :there
    (<= val_1329 7) :marty))