clojure.core - into

f i want to conj a bunch of objects onto a vector in a map atom, and i have them in a list, is it better to write a (doseq [obj objects] (swap! v update :key conj obj) or something like (swap! v update :key #(apply conj % objects))?

that's a cumbersome way of saying: is it "better"/more idiomatic to do a bunch of swap!s or a single swap! with apply?

absolutely the latter
the former is N separate transactions (and someone else could be doing the same thing, interleaving the results)
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but really you'd probably be better off with (swap! v update :key into objects)
but be careful with the nil case - into uses a list in that case
I guess conj has the same problem (since into uses conj)
into will also automatically use transients if it can (like with vectors)

guard against the nil case with fnil like so: (swap! v update :key (fnil into []) objects), to make the type of the collection explicit.
Can easily swap out the vector in (fnil into []) for a #{} or {} depending on the type one wants.

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