On Wed, Feb 9, 2011 at 6:07 AM, Andreas Kostler
<andreas.koestler.le...@gmail.com> wrote:
> Hello all,
> I've come across the following macro, which basically tries to execute body
> and times out after ms milliseconds. It works fine :)
>
> (defmacro time-limited [ms & body]
> `(let [f# (future ~@body)]
> (. get f# ~ms java.util.concurrent.TimeUnit/MILLISECONDS)))
>
> If I do a (macroexpand '(time-limited 100 (println "Hello World"))) it
> expands to:
> (let* [f__1760__auto__ (clojure.core/future (println "Hello"))] (.get
> f__1760__auto__ 100 java.util.concurrent.TimeUnit/MILLISECONDS))
>
> My questions are:
> 1) Where does the let* come from?!?
The clojure.core/let is a macro that expands into let*, which is the
true special form and doesn't do destructuring.
Use macroexpand-1 instead of macroexpand to avoid that cruft.
> 2) Why does f need to be a auto generated variable? Wouldn't (let* [f
> (clojure.core/future (println "Hello"))] (.get f 100
> java.util.concurrent.TimeUnit/MILLISECONDS)) suffice?
It would crap out if you did something like:
(let [f (my-thingie)]
(time-limited 1000
(adjust-thingie f)))
Specifically adjust-thingie would throw a ClassCastException
resembling this one:
#<CompilerException java.lang.ClassCastException:
clojure.core$future_call$reify__5500 cannot be cast to
java.lang.Number (foo.clj:112)>
It would probably not be very obvious why that was happening, either.
> 3) What's .get ??
A Java method call on the future object.
user=> (parents (type (future 17)))
#{java.util.concurrent.Future
clojure.lang.IDeref
clojure.lang.IObj
java.lang.Object}
A Clojure future is just a Java future with a couple of Clojure
interfaces slapped on to allow @foo and metadata to be used with it.
@foo and (deref foo) don't let you specify a timeout, though; the Java
Future.get() method does.
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