core.sync completely trashes the stack in an exception. Print any exception to the console before that happens.

src/re_frame/handlers.cljs

@@ -65,5 +65,13 @@
         handler-fn  (lookup-handler event-id)]
     (if (nil? handler-fn)
       (warn "re-frame: no event handler registered for: \"" event-id "\". Ignoring.")   ;; TODO: make exception
-      (handler-fn app-db event-v))))
+      (try
+        (handler-fn app-db event-v)
+        (catch :default e
+          (do
+            ;; use of a core.async loop seems to completely ruin exception stacks.
+            ;; So we're going print the exception to the console here, before it gets trashed.
+            (.error js/console e)
+            (throw e)))))))
+
 

src/re_frame/router.cljs

@@ -32,10 +32,7 @@
                _        (if (:flush-dom (meta event-v))   ;; check the event for metadata
                           (do (flush) (<! (timeout 20)))  ;; wait just over one annimation frame (16ms), to rensure all pending GUI work is flushed to the DOM.
                           (<! (timeout 0)))]              ;; just in case we are handling one dispatch after an other, give the browser back control to do its stuff
-           (try
-             (handle event-v)                             ;; XXX There should be a plugable place to write exceptions and warns
-             (catch js/Object e
-               (.error js/console e)))  ;; exceptions are a special kind of rubbish when you are in a goloop. So catch it here and print.
+           (handle event-v)
            (recur)))
 
 
@@ -55,8 +52,13 @@
 
 
 (defn dispatch-sync
-  "Invoke the event handler sycronously, avoiding the async-inducing use of core.async/chan"
+  "Send an event to be processed by the registered handler, but avoid the async-inducing
+  use of core.async/chan.
+
+  Usage example:
+     (dispatch-sync [:delete-item 42])"
   [event-v]
-  (handle event-v))
+  (handle event-v)
+  nil)    ;; Ensure nil return. See https://github.com/Day8/re-frame/wiki/Returning-False