Quickly look at a core.async channel with only momentary blocking, pulls the data if it exists and thus alters the channel.Drain! will repeatedly call peek! until a maximum number of elements is pulled or until the channel is empty.

peek-drain.clj

;;uses core.async and refers chan, >!, go, alts!! and timeout functions

(def c (chan))

(go (>! c {:msg "hi"}))

(defn peek! [ch & args]
  "quickly looks at a channel for new incoming data, pulls it if it exists, quits otherwise;
returns seq if specifying max elements to pull"
  (if-not (nil? args)
    (let [s (remove nil? (repeatedly (first args) #(peek! ch)))] (if-not (empty? s) s nil))
    (let [[m ch] (alts!! [ch (timeout 0)])] m)))
	
(peek! c) ;; {:msg "hi"}


(go (>! c {:msg "hi"}))
(go (>! c {:msg "hey"}))
(go (>! c {:msg "hello"}))
(go (>! c {:msg "holla"}))

(peek! c 20)