async_trampoline.clj

  "X1. A drinks dispenser has two buttons labelled ORANGE and
  LEMON. The actions of pressing the two buttons are /set-orange/ and
  /set-lemon/. The actions of dispensing a drink are /orange/ and
  /lemon/. The choice of drink that will be dispensed is made by
  pressing the corresponding button. Before any button is pressed, no
  drink will be dispensed."
  (declare orange lemon)

  (def c (chan))

  (defn orange []
    (println "Dispenser is set to orange")
    (go
      ;; An orange drink is taken from the dispenser or
      ;; the dispenser's setting is changed
      (let [[v p] (alts! [[c :orange] c])]
        (condp = v
          :set-lemon  #(lemon)
          :set-orange #(orange)
          #(orange)))))

  (defn lemon []
    (println "Dispenser is set to lemon")
    (go
      ;; A lemon drink is taken from the dispenser or
      ;; the dispenser's setting is changed
      (let [[v p] (alts! [[c :lemon] c])]
        (condp = v
          :set-orange #(orange)
          :set-lemon  #(lemon)
          #(lemon)))))

  (defn drink-dispenser []
    (println "Starting drink dispenser")
    (go
      (let [x (<! c)]
        (condp = x
          :set-orange (trampoline orange)
          :set-lemon  (trampoline lemon)))))

  (drink-dispenser)
  ;; => Starting drink dispenser
  (go (>! c :set-orange))
  ;; => Dispenser is set to orange
  (go (println "Retrieving:" (<! c)))
  ;; this expression parks waiting for a `>!`
  ;; but `orange` is parked at `alts!` waiting to see if `(>! c :orange)` succeeds