Add log-ex middleware.

src/re_frame/handlers.cljs

@@ -65,13 +65,5 @@
         handler-fn  (lookup-handler event-id)]
     (if (nil? handler-fn)
       (warn "re-frame: no event handler registered for: \"" event-id "\". Ignoring.")   ;; TODO: make exception
-      (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.stack)
-            (throw e)))))))
-
+      (handler-fn app-db event-v))))
 

src/re_frame/middleware.cljs

@@ -35,6 +35,25 @@
           (if-not (identical? db new-db)
             (reset! app-db new-db)))))))
 
+(defn log-ex
+  "Middleware which catches and prints any handler-generated exceptions to console.
+  Handlers are called from within a core.async go-loop, and core.async produces
+  a special kind of hell when in comes to stacktraces. By the time an exception
+  has passed through a go-loop its stack is mangled beyond repair and you'll
+  have no idea where the exception was thrown.
+  So this middleware catches and prints to stacktrace before the core.async sausage
+  machine has done its work.
+  "
+  [handler]
+  (fn log-ex-handler
+    [db v]
+    (try
+      (handler db v)
+      (catch :default e     ;; ooops, handler threw
+        (do
+          (.error js/console e.stack)
+          (throw e))))))
+
 
 (defn debug
   "Middleware which logs debug information to js/console for each event.