This is quite amusing since the first reply to the original post already provided the correct answer :) On Sep 28, 2011 6:05 AM, "Michał Marczyk" <michal.marc...@gmail.com> wrote: > Hi Ru, > > let's input your macro definition at the REPL: > > user> (defmacro infix [e] `(let [[x# f# y#] ~e] (f# x# y#))) > #'user/infix > > So far so good. Now let's try use it in a function: > > user> (defn foo [] (infix (5 + 4))) > #'user/foo > > Well now -- it compiled! So, there's no exception being thrown when > the macro is expanded at compile time; otherwise foo would not have > compiled. > > How about calling foo? > > user> (foo) > ; Evaluation aborted. > user> *e > #<ClassCastException java.lang.ClassCastException: java.lang.Long > cannot be cast to clojure.lang.IFn> > > There's your exception: at runtime. By this time there is no trace of > your macro in the running code (you could undefine it -- by saying > (ns-unmap 'user 'infix) -- and this would have no effect on foo). > > > Once again: (5 + 4) *is not evaluated when the macro is expanded*. It > is only evaluated at runtime -- and only then does it explode, as > expected. The key point is that a macro is just a function called upon > by the compiler to transform your program prior to it being compiled > into JVM bytecode (in the case of Clojure, or perhaps native code in > the case of some Common Lisp implementations and execution by the > interpreter in interpreted Lisps); if it generates erroneous code > (like this version of infix!), that erroneous code will be compiled by > the compiler and eventually explode when you run it -- an unpleasant > occurrence completely distinct from a macro-expansion-time exception. > > Sincerely, > Michał > > -- > 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
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