Explain why event-fx handlers can't return cofx maps

docs/FAQs/use-cofx-as-fx.md

@@ -0,0 +1,47 @@
+<!-- leave this H1 here. It stops mkdocs putting in a Title at the top.
+     It needs to be at the top of the file otherwise it breaks the 
+     table of contents on the right hand side. -->
+#
+
+## Question
+
+With `reg-event-fx`, why can't my handler function just update the first argument (i.e. the `cofx` map) and pass it on? 
+
+When I try, I get a warning, such as: `no handler registered for effect: :event. Ignoring.`
+
+## Answer
+
+Effects simply aren't coeffects.
+
+To fix this warning, just declare a new map of effects. For instance:
+
+```clj
+(reg-event-fx ::cow-clicked
+  (fn [{:keys [db] :as cofx} _]
+    {:db (update db :clicks inc)}))
+```
+
+## Context
+
+This seems like it would work, if you're used to using `reg-event-db` this way.
+Such a `db` handler behaves like a reducing function:
+
+```clj
+(reg-event-db ::cow-clicked-db
+  (fn [db _] (update db :clicks inc)))
+```
+
+Doing a similar thing to a coeffect map causes the warning, however, not to mention possible unintended effects:
+
+```clj
+(reg-event-fx ::cow-clicked-bad
+  (fn [cofx _] (update-in cofx [:db :clicks] inc)))
+```
+
+With `reg-event-fx`, you receive a map of coeffect values, and you're expected to return a map of effect values. 
+It's a coincidence that `:db` is the name of both an effect and a coeffect.
+But re-frame adds other coeffects, such as `:event`, which it does *not* consider effects.
+
+So your handler ends up doing `(update-in {:db {:clicks 0} :event [::cow-clicked-bad] :your-other-coeffect 25} ...)`. 
+
+Any coeffects you declare cause this problem, too. You might have a coeffect, like `:db`, which shares its name with an effect. But that's a deliberate design decision, not a default.

mkdocs.yml

@@ -84,6 +84,7 @@ nav:
     - "Why is my input field laggy?": FAQs/laggy-input.md
     - "How long after I do a dispatch does the event get handled?": FAQs/When-Does-Dispatch-Happen.md
     - "How can I use a subscription in an Event Handler": FAQs/UseASubscriptionInAnEventHandler.md
+    - "Why can't I turn a `cofx` map into an `fx` map?": FAQs/use-cofx-as-fx.md
     - "How do I use logging method X": FAQs/Logging.md
     - "Dispatched Events Are Null": FAQs/Null-Dispatched-Events.md
     - "Why do we need to clear the subscription cache when reloading with Figwheel?": FAQs/Why-Clear-Sub-Cache.md

src/re_frame/fx.cljc

@@ -56,7 +56,13 @@
                 (doseq [[effect-key effect-value] effects-without-db]
                   (if-let [effect-fn (get-handler kind effect-key false)]
                     (effect-fn effect-value)
-                    (console :warn "re-frame: no handler registered for effect:" effect-key ". Ignoring."))))))))
+                    (console :warn
+                             "re-frame: no handler registered for effect:"
+                             effect-key
+                             ". Ignoring."
+                             (when (= :event effect-key)
+                               (str "You may be trying to return a coeffect map from an event-fx handler. "
+                                    "See https://day8.github.io/re-frame/use-cofx-as-fx/"))))))))))
 
 ;; -- Builtin Effect Handlers  ------------------------------------------------