in response to http://oobaloo.co.uk/multiplexing-work-reliably-and-efficiently-with-state-machines-and-core-dot-async .... so (start) will give you a channel to put initial states on.. or you can comment-in line 70

progressor.clj

(ns async-state-progressor.progressor
  (:require [clojure.core.async :refer [go-loop >! <! thread chan timeout onto-chan]]))

(defn log-state [msg current-state]
  (locking *out* (println msg (:state current-state))))

(defn download-report [{:keys [client date] :as state}]
  ;; snip
  (assoc state :file "dummy-file"))

(defn upload-to-s3 [{:keys [file] :as state}]
  ;; snip
  (assoc state :bucket "dummy-bucket" :key "dummy-key"))

(defrecord Transition [op next-state])

(defn completed? [{:keys [state]}]
  (= :completed state))

(defn transition [{:keys [state] :as current-state}]
  (log-state "transition from " current-state)
  (condp = state
    :downloadable (Transition. download-report :uploadable)
    :uploadable (Transition. upload-to-s3 :completed)))

(defn step [current-state]
  (let [{:keys [op next-state]} (transition current-state)]
    (try (assoc (op current-state) :state next-state)
         (catch Exception e
           (assoc current-state :error e)))))



(defn progressor [states-ch completed-ch]
  (go-loop [current-state (<! states-ch)]
    (log-state "current-state: " current-state)
    (if (completed? current-state)
      (do (log-state "sending completed.. " current-state) (>! completed-ch current-state))
      (let [{:keys [error] :as new-state} (<! (thread (step current-state)))]
        (log-state "new-state: " new-state)
        (if error
          (do (println "Error: " (:error new-state))
              (<! (timeout 5000))                           ; delay the retry for 5s
              (>! states-ch current-state))
          (and new-state (do (log-state "sending state.. " new-state) (>! states-ch new-state))))))
    (recur (<! states-ch))))


(defn initial-state [date client]
  {:date   date
   :client client
   :state  :downloadable})

(def completed-ch (chan))

(def initial-states (repeat 42 (initial-state "dummy-date" "dummy-client")))

(defn start []
  (let [states-ch (chan 1024)]

    (doseq [_ (range 10)]
      (progressor states-ch completed-ch))

    ;; snip- creates sequence of initial states
    ;; using initial-state fn.

    #_(doseq [state initial-states]
      (>!! states-ch state))

    (onto-chan states-ch (repeat 15 (initial-state "dummy-date" "dummy-client")) false)

    ;; snip- wait for all operations to complete
    (go-loop [compl (<! completed-ch)]
      (log-state "COMPLETE: " compl)
      (recur (<! completed-ch)))

    states-ch))

I am getting this error:

Assert failed: Can't recur here at frame at line 46.

This is with the following project.clj:

(defproject progressor "0.1.0-SNAPSHOT"
:description "FIXME: write description"
:url "http://example.com/FIXME"
:license {:name "Eclipse Public License"
:url "http://www.eclipse.org/legal/epl-v10.html"}
:dependencies [[org.clojure/clojure "1.6.0"]
[org.clojure/core.async "0.1.346.0-17112a-alpha"]])

I haven't really looked into it any more.

can you retry ?
sorry, messed up the comments... it should use core async's "onto-chan" instead of the (doseq ...) part (which also works, if you comment-in)