Those are very good questions, in the sense that I'm not sure of the answers to them yet :)

First the basic problem: since seqs in general are lazy, they are typically not evaluated in the function where they are declared. So if you have something like

(def foo (atom ["hello"]))

(defn bar []
 [:div
  (for [i (range 1)]
    [:p (@foo i)])])

then foo would never be deref'ed when bar is run, but rather inside the :p component (or possibly the :div). Then obviously bar doesn't know that it needs to re-run when foo has changed, and general badness ensue.

This could be solved for most cases, by keeping track of the current "owner" component, and pass that along to children, so that when the seq is eventually evaluated, a dependency is added to, say, bar. But getting that right for all cases is kind of a bitch. :)

And I can't really think of a case where you wouldn't be better off lifting the deref out of the for loop. That is better for performance, and makes it more obvious what happens.

The exception vs. warning question is also a good one. bar would for example have been perfectly safe it it had looked like:

(defn bar []
 @foo
 [:div
  (for [i (range 1)]
    [:p (@foo i)])])

Then bar is re-rendered when foo changes, and everything works just fine. You just lose a tiny bit of performance from the extra deref. So maybe in this case it would have been a bit harsh to throw an exception – but I'm not 100% sure about that...

There can be other cases as well, if (for example) you're perfectly ok with the seq not being updated when the atom changes. I actually had that very case in the color palette demo here: http://holmsand.github.io/reagent/news/reagent-is-async.html

But even in that case, I would have been better off using explicit values, so the demo has now been changed – so in that case I appreciated the warning :)

So I'm still unsure if it is actually worth the extra complexity to allow derefs in seqs, and if it should be warning or exception when it happens. And since I'm not 100% sure, a warning seemed like the right thing to do.

Brilliant ideas are very much welcome. And maybe the warning could have better wording as well.

You can also prepend the 'for statement with 'doall

(defn bar []
 [:div
  (doall (for [i (range 1)]
    [:p (@foo i)]))])

Could you just do

(into [:div]
  (for [i (range 1)]
     [:p (@foo i)]))