A Dataflow Engine

                                  by

                          Jeffrey Straszheim

                                   
This package is a dataflow engine for Clojure modeled loosely on the
"Cells" model, but using Clojure Ref's to manage state rather than
Agents.

Usage:

  (def df
   (build-dataflow
    [

     ; Source cells are set by the user
     (cell :source fred 1)

     ; Reference other cells by ?name
     (cell greg (+ ?fred ?mary))

     (cell joan (+ ?fred ?mary))

     ; Order does not matter
     (cell :source mary 0)

     (cell joan (* ?fred ?mary))

     ; Multiple joans exist, ?*name returns a collection
     (cell sally (apply + ?*joan))

     ; validators can return exceptions.  ?-name gets the old value
     (cell :validator (when (number? ?-greg)
                        (when (<= ?greg ?-greg)
                          (throwf Exception "Non monotonic"))))]))

  (add-cell-watcher (get-cell df 'sally)
                    nil
                    (fn [key cell o n]
                      (printf "sally changed from %s to %s\n" o n)))

  (full-update df)
  (update-values df {'fred 1 'mary 1})
  (update-values df {'fred 5 'mary 1})
  (update-values df {'fred 0 'mary 0}) ; Will throw Non monotonic exception

  (get-value df 'fred)
  (get-values df 'joan)
  (get-value df 'sally)
  (get-value df 'greg)