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;;; auto_agent.clj: agents that update automatically from a formula
;; by Stuart Sierra, http://stuartsierra.com/
;; January 29, 2009
;; Copyright (c) Stuart Sierra, 2009. All rights reserved. The use
;; and distribution terms for this software are covered by the Eclipse
;; Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php)
;; which can be found in the file epl-v10.html at the root of this
;; distribution. By using this software in any fashion, you are
;; agreeing to be bound by the terms of this license. You must not
;; remove this notice, or any other, from this software.
;; This is superficially similar to Ken Tilton's "Cells" library for
;; Common Lisp. But Cells is single-threaded and synchronous. This
;; version is built on Clojure agents, so it is multi-threaded and
;; asynchronous.
;;
;; An auto-agent is an agent whose value is the result of an
;; expression. That expression can include other mutable Clojure
;; types -- agents, atoms, and refs -- dereferenced with "deref" or
;; "@". Whenever one of those derefernced things changes, the
;; auto-agent will be automatically updated to reflect the new value.
(ns clojure.contrib.auto-agent
(:use [clojure.contrib.walk :only (macroexpand-all)]
[clojure.contrib.test-is :only (deftest- is)]))
(defmacro #^{:private true} find-derefs
"Expands the expression and returns the set of all agents/atoms/refs
dereferenced within it."
[expr]
(let [expr (macroexpand-all expr)]
(if (coll? expr)
(set (map second
(filter #(and (list? %)
(symbol? (first %))
(= (resolve (first %)) #'clojure.core/deref))
(tree-seq coll? seq expr))))
#{})))
(defmacro auto-agent
"Creates an agent whose value is the result of evaluating expr.
Whenever one of the agents/atoms/refs dererenced within expr
changes, expr is reevaluated and this agent's state is set to its
new value."
[expr]
`(let [value-fn# (fn [old# a#] ~expr)
cell# (agent (value-fn# nil nil))]
(doseq [d# (find-derefs ~expr)]
(add-watcher d# :send cell# value-fn#))
cell#))
;;; TESTS
;; Run these tests with
;; (clojure.contrib.test-is/run-tests 'clojure.contrib.auto-agent)
;; Bind clojure.contrib.test-is/*load-tests* to false to omit these
;; tests from production code.
(deftest- auto-agents-can-depend-on-agents
(let [c1 (agent 1)
c2 (auto-agent (inc @c1))]
(is (= 1 @c1))
(is (= 2 @c2))
(send c1 inc)
(await c1 c2)
(is (= 2 @c1))
(is (= 3 @c2))))
(deftest- auto-agents-can-depend-on-refs
(let [c1 (ref 1)
c2 (auto-agent (inc @c1))]
(is (= 1 @c1))
(is (= 2 @c2))
(dosync (alter c1 inc))
(await c2)
(is (= 2 @c1))
(is (= 3 @c2))))
(deftest- auto-agents-can-depend-on-atoms
(let [c1 (atom 1)
c2 (auto-agent (inc @c1))]
(is (= 1 @c1))
(is (= 2 @c2))
(swap! c1 inc)
(await c2)
(is (= 2 @c1))
(is (= 3 @c2))))
(deftest- very-long-dependency-chains-work-with-agents
(let [head (agent 1)
chain (take 10000 (iterate (fn [p] (auto-agent (inc @p))) head))
tail (auto-agent (inc @(last chain)))]
(is (= 1 @head))
(is (= 10001 @tail))
(send head inc)
;; We can't just do (apply await head tail chain) because not all
;; the actions have been sent by the time we call await. Instead,
;; we have to await each cell in order. In general, you should
;; not rely on agent cell updates happening at any specific time.
;; This test is only to demonstrate that very long chains of agent
;; cells can be made without causing a stack overflow, which is
;; not true of ref/atom cells.
(await head)
(doseq [c chain] (await c))
(await tail)
(is (= 2 @head))
(is (= 3 @(second chain)))
(is (= 10001 @(last chain)))
(is (= 10002 @tail))))
(deftest- agent-expressions-can-be-vectors
(let [c1 (agent 1)
c2 (agent 2)
c3 (auto-agent [@c1 @c2])]
(is (= [1 2] @c3))
(send c1 inc)
(send c2 inc)
(await c1 c2)
(await c3)
(is (= [2 3] @c3))))
(deftest- agent-expressions-can-be-maps
(let [c1 (agent 1)
c2 (agent 2)
c3 (auto-agent {:c1 @c1, :c2 @c2})]
(is (= {:c1 1, :c2 2} @c3))
(send c1 inc)
(send c2 inc)
(await c1 c2)
(await c3)
(is (= {:c1 2, :c2 3} @c3))))
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