kevin.clj

(declare *fails* *succeeds* *exits*)
(def *horizon* nil)

(defmacro when-hrz [which & forms]
  `(do
     (when (not *horizon*) (throw (RuntimeException. "Naked Singularity")))
     (swap!
       ~(condp = which :fails '*fails* :succeeds '*succeeds* :exits '*exits*)
       conj (fn [] ~@forms))))

(defmacro horizon [& body]
  `(binding [*horizon* true
             *fails* (atom (list))
             *succeeds* (atom (list))
             *exits* (atom (list))]
            (try
              (let [y# (do ~@body)] (dorun (map (fn[x#](x#)) @*succeeds*)) y#)
              (catch Exception e# (dorun (map (fn[x#] (x#)) @*fails*))
                     (throw e#))
              (finally
                (dorun (map (fn[x#] (x#)) @*exits*))))))

(defn foo []
  (when-hrz :fails (prn "failed"))
  (when-hrz :succeeds (prn "succeeded"))
  (when-hrz :exits (prn "exited")))

(horizon
 (foo)
 42)

(horizon
 (foo)
 (throw (Exception. "err"))
 42)