I'd like to focus the attention back to a specific example. See the
first code snippet at http://clojure.org/concurrent_programming.
(import '(java.util.concurrent Executors))
(defn test-stm [nitems nthreads niters]
(let [refs (map ref (replicate nitems 0))
pool (. Executors (newFixedThreadPool nthreads))
tasks (map (fn [t]
(fn []
(dotimes [n niters]
(dosync
(doseq [r refs]
(alter r + 1 t))))))
(range nthreads))]
(doseq [future (. pool (invokeAll tasks))]
(. future (get)))
(. pool (shutdown))
(map deref refs)))
Many people new to Clojure will be attracted to this page because they
will have heard that one of the strengths of Clojure is how good it is
for concurrent programming.
I believe the purpose of this example is to demonstrate how much
easier it is to program with Clojure refs instead of Java locks.
Certainly this code is much shorter than similar Java code. There is a
paragraph before the code that explains what the code does and there
are no comments within the code.
Suppose you had been studying Clojure for one week before coming
across this code. Would you know what was going on here? Let's see ...
we've got an anonymous function that uses an anonymous function which
iterates some number of times calling dosync on a doseq ... alter
changes the value of a ref ... it uses a future ... My fear is that
many developers will become discouraged and stop trying to learn
Clojure, fearing that it's just too hard. I think we need to work to
minimize those kinds of reactions to sample code. Adding some comments
to non-obvious code is one way to do that.
I don't mean to single out this one example. Many examples of Clojure
code seem equally daunting to me and most Clojure code seems to
contain no comments at all.
--
R. Mark Volkmann
Object Computing, Inc.
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