Hi Ken, Thanks again. However, I still have one nagging gap in understanding this macro. If I replace
(defmacro time-limited [ms & body] `(let [f# (future ~@body)] (. get f# ~ms java.util.concurrent.TimeUnit/MILLISECONDS))) which expands to: (macroexpand-1 '(time-limited 1000 (println "Hello"))) (clojure.core/let [f__1760__auto__ (clojure.core/future (println "Hello"))] (.get f__1760__auto__ 1000 java.util.concurrent.TimeUnit/MILLISECONDS)) with: (defmacro time-limited-2 [ms & body] `(let [f (future ~@body)] (. get f ~ms java.util.concurrent.TimeUnit/MILLISECONDS))) which expands to: (macroexpand-1 '(time-limited-2 1000 (println "Hello"))) (clojure.core/let [user/f (clojure.core/future (println "Hello"))] (.get user/f 1000 java.util.concurrent.TimeUnit/MILLISECONDS)) So far so good. Now time-limited-2 bombs out with the following exception: Can't let qualified name: user/f [Thrown class java.lang.Exception] Why is that? >> >> (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. > > -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with your > first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group at > http://groups.google.com/group/clojure?hl=en -- “There is a strong correlation between being smart and being a nerd, and an even stronger inverse correlation between being a nerd and being popular” (Paul Graham) -- ********************************************************** Andreas Koestler, Software Engineer Leica Geosystems Pty Ltd 270 Gladstone Road, Dutton Park QLD 4102 Main: +61 7 3891 9772 Direct: +61 7 3117 8808 Fax: +61 7 3891 9336 Email: andreas.koest...@leica-geosystems.com ************www.leica-geosystems.com************* when it has to be right, Leica Geosystems Please consider the environment before printing this email. -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en