Dear Michal, You and others explain me how Clojure (or Common Lisp) handles my code. Very detailed and thoroughly. I want to draw attention to another point: The documentation says that the macro function is called with the arguments unevaluated. So I write code for that function on that assumption. And when I run this code the arguments being evaluated. It does not matter why or how and when! They mustn't evaluated never in that case in accordance with the documentation! That is the question! Because the unevaluated arguments is an important feature that allows you to expand the language using macros.
On 28 сен, 00:35, 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
