Hi Jan, OK, I get it, thanks a lot for your quick answer. Eric Le samedi 15 février 2014 17:49:57 UTC+1, Jan Herich a écrit :
> Hello Eric, > > You can rewrite this functionality with key# and r# instead of ~'key and > ~'r and it would work just as well, > it's only not necessary to use unique symbols here, because you are not > using them in the function body > anyway, so there is no danger of accidental var capture. > > To be honest, i would rather use underscore symbols (written as ~'_ inside > macro definition) to indicate > that i won't use any of the first two parameters in the function body. > > Dňa sobota, 15. februára 2014 9:32:48 UTC+1 Eric napísal(-a): >> >> Hi, >> >> I'm reading the second edition of Joy of Clojure (the MEAP), and there is >> an example that I don't quite get, and I was hoping someone here could help >> me. It's in chapter 8, talking about macros. There is an example of a macro >> called def-watched, which prints a message each time the root binding of a >> var changes: >> >> (defmacro def-watched [name & value] >> `(do >> (def ~name ~@value) >> (add-watch (var ~name) >> :re-bind >> (fn [~'key ~'r old# new#] >> (println old# " -> " new#))))) >> >> I understand almost everything, but I don't see why we have to use ~'keyand >> ~'r rather than simply key# and r#. As I understand it, the first option >> (~'var-name) would be useful to capture an external var inside the >> macro, but I don't see the point of doing it here, especially since ~'keyis >> defined in the parameter list, so that it would anyway hide any external >> var. Could someone please help me understand this? >> >> Thank you in advance, >> >> Eric >> > -- 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 --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
