Hi all, quick question. What is the impact on the store if reg-event-fx returns a nil value? I have a reg-event-fx which uses an if-let. I noticed with reg-event-db that my db value was "niled" if there was an if which returned nil.

I think you’ll want to return the unchanged db in your reg-event-fx {:db db} rather than having it return nil.

same with reg-event-db, you’ll want to return the existing db rather than a nil value in case of a no-op

forgot if you can omit the db key from reg-event-fx, might want to try that out

from my experience you can return nil

yeah just testing it out, reg-event-fx doesn’t need {:db db}, you can just return nil and no side effects get called.

Ok, great. Yea, I noticed that as well and wanted to double check 😄 So, for reg-event-db we have to return {:db db} and reg-event-fx we can return nil with no issues. Is this in the docs somewhere?

i think the way to look at it is that reg-event-fx takes a map of coeffects to perform

reg-event-db’s job is only to perform the :db coeffect

which swaps the existing db with the one provided

https://github.com/Day8/re-frame/blob/master/docs/Coeffects.md

To make this common case easy to program, there's a specific registration function, called reg-event-db, which delivers ONLY the coeffect db to the event handler (and event of course).

I found the way reg-fx is used to setup the :db effect handler was useful to show how reg-fx works: https://github.com/day8/re-frame/blob/master/src/re_frame/fx.cljc#L160