Howdy --
Trying to use core.async's mix facility, one difficulty I've run into is
the lack of a means to close the output channel only after al inputs have
been exhausted. In the interim, I've ended up using the below instead --
which lacks some of the facilities provided by the mix interface (muting,
pausing, soloing, etc), but has the advantage of cleanly closing its output
channel if and only if all inputs (including the channel from which other
channels are read) have been flushed.
That said, looking at core.async's code, it certainly seems possible to
extend the mix abstraction to handle this use case -- for instance, adding
a (close-on-empty! [mix]) call which would set a flag causing the the
output channel to close whenever all inputs are unmixed, whether by user
interaction or end-of-input, would be another solution.
Thoughts? Should I publish the below (and related tooling -- such as an
N-way split helper used to create such channels of channels) as a separate
library? Submit patches to core.async adding appropriate functionality to
the mix implementation? Or is there already a better way of achieving the
desired effect?
Thanks!
(defn merge-channels
"Given a channel which yields other channels, combine output from all of
these into a single output channel.
Close output channel and exit when both the input channel and all
channels read from that channel have closed."
[in-chan]
(let [out-chan (chan)]
(go
(loop [open-channels #{}, in-chan-open? true]
(let [all-chans (if in-chan-open?
(conj open-channels in-chan)
open-channels)
[v c] (when-not (empty? all-chans) (alts! (vec all-chans)))]
(cond
(empty? all-chans)
nil
(= c in-chan)
(if v
(recur (conj open-channels v) true)
(recur open-channels false))
(nil? v)
(do
(recur (disj open-channels c) in-chan-open?))
:else
(do
(>! out-chan v)
(recur open-channels in-chan-open?))))))
out-chan))
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