I see. That section is just horribly written, calling something a macro when it isn't one yet. Here's my thinking in **bold**, inlined.
Simple Macro If you understand the reader and evaluator, there actually isn't all that much more to understand about the operation and creation of macros, for a macro is simply a regular function that is called in a special way. When called as a macro, a function takes its arguments *un*evaluated and returns an expression to be evaluated in place of the call to the macro. A very simple (and pointless) macro would be one that simply returns its argument. **Here's a first attempt which is NOT a macro:** (def pointless (fn [n] n)) Whatever is passed to this macro---a list, a symbol, whatever---will be returned unmolested and then evaluated after the call. **At least, that's what we want to happen, and that's what would happen to a real macro. But this is not a real macro yet.** Effectively, calling this macro is pointless:* *And if this were a real macro, this is what would happen:** (pointless (+ 3 5)) ; pointless returns the list (+ 3 5), which is then evaluated in its place(+ 3 5) ; may as well just write this instead But as we defined *pointless* above, it is just a regular function, not a macro. **The argument to pointless (+ 3 5) gets evaluated to 5 and then enters the body of the function, which simply returns 5.** To make it a macro, we need to attach the key-value pair :macro true as metadata to the Var mapped to *pointless* by the def. There are a number of ways to do this, but most commonly we would simply define the function as a macro with the provided macro *clojure/defmacro*: (defmacro pointless [n] n) ; define a macro pointless that takes one parameter and simply returns it **...without evaluating it -- now it behaves as we intended in the previous code snippet on this page** -- 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
