Hi all,
I thought I'd share the results of a fun little excercise I did this
morning: running Clojure on a VM with native support for tail call
optimization and first class continuations. Any Schemers on this list
will probably appreciate it.
The VM in question is Avian (http://oss.readytalk.com/avian/), built with
optional tail call and continuation features enabled and using the OpenJDK
class library. It's not nearly as fast or sophisticated as e.g. Hotspot,
but it has some features that make it interesting for running non-Java
languages. I'm using the latest commit from the Git repository.
For example, here's what happens if you recurse too deeply using Hotspot:
$ java -cp clojure.jar clojure.main
Clojure 1.2.1
user=> ((fn this [n] (if (= n 1000000) n (this (+ n 1)))) 0)
java.lang.StackOverflowError (NO_SOURCE_FILE:0)
user=>
With Avian, there is no overflow, and we also get to play with
continuations:
$ ../avian/build/linux-x86_64-tails-continuations-openjdk-src/avian -Xmx512m
-cp clojure.jar clojure.main
Clojure 1.2.1
user=> ((fn this [n] (if (= n 1000000) n (this (+ n 1)))) 0)
1000000
user=> (defn call-cc [function]
(avian.Continuations/callWithCurrentContinuation
(reify avian.CallbackReceiver
(receive [_ continuation]
(function (fn [result] (.handleResult continuation result)))))))
#'user/call-cc
user=> (call-cc (fn [continuation] (continuation "hello, world!")))
"hello, world!"
user=>
For more information about how continuations work in Avian, please see
http://oss.readytalk.com/avian/javadoc/avian/Continuations.html.
Anyway, I think this is an interesting experiment since it shows that
if/when tail call and continuation support are integrated into the JVM
officially (http://openjdk.java.net/projects/mlvm/), Clojure could benefit
with few or no modifications.
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