On Fri, Jan 6, 2012 at 11:12 AM, Andrew <ache...@gmail.com> wrote: > http://en.wikibooks.org/wiki/Learning_Clojure/Macros > > The page says the following: > > (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. Effectively, calling > this macro is pointless:" > > (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 actually, doesn't (+ 3 5) get evaluated before pointless ever sees it?
No. Macro arguments are passed as unevaluated forms. However, numbers and data structures are still usable. If you pass 3 to a macro you can add to it, check that it's odd, etc. in the macro; if you pass [7 8 (* 4 5)] the macro code sees, and can traverse, a vector with the integers 7 and 8 in it -- but the last component of the vector will be the list '(* 4 5) rather than the number 20. The macro can always emit code that will process the evaluated items. For example, it can emit `(reduce + ~v) with v having been that vector, and the output of '(reduce + [7 8 (* 4 5)]) will evaluate to 35, as it should. -- 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
