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:

(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:

(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:

(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 identifies 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} ...).

There's probably no effect handler for :event. Hence the warning. Coeffects which you declare, such as your-other-coeffect, may cause the same problem.

You might have a coeffect, similar to :db, which shares its name with an effect. But that's a deliberate design decision, not a default.