On Fri, Jun 7, 2013 at 11:14 AM, Ben Wolfson <wolf...@gmail.com> wrote:
> On Fri, Jun 7, 2013 at 11:09 AM, larry google groups < > lawrencecloj...@gmail.com> wrote: > >> >> I am very stupid and I am having trouble figuring out how to read >> this: >> >> (defmacro match-func [& body] `(fn [~'x] (match [~'x] ~@body))) >> >> ((match-func [q :guard even?] (+ 1 q) [z] (* 7 z)) 33) >> ;; 231 >> >> What? Why 231? >> > > Because 7 * 33 = 231. > > The macro (which IMO is terrible and shouldn't be emulated) > Why do you think the macro is terrible? > creates a function of one argument that matches its argument with > match/action expressions given in its invocation. So > > (match-func [q :guard even?] (+1 q) [z] (* 7 z)) > > expands into > > (fn [x] > (match [x] > [q :guard even?] (+ 1 q) > [z] (* 7 z))) > > i.e., match the argument with q if it's even and return (+ 1 q); > otherwise, match it with z and return (* 7 z). Since 33 is odd, you get 231. > > In general, using macroexpand-1 is very useful for figuring out what's > happening with a macro. > > > >> The article is here: >> >> http://java.dzone.com/articles/my-first-clojure-macro >> >> I especially struggle with the longer version: >> >> (defmacro match-pfunc [& body] >> "Create a partial function that does pattern matching." >> (let [rewrite (mapcat (fn [x] [(first x) true]) (partition 2 body))] >> `(fn ([x#] (match [x#] ~@body)) >> ([x# y#] >> (cond >> (= :defined? x#) >> (match [y#] ~@rewrite) >> (= :body x#) >> '(~@body)))))) >> >> >> The article says: >> >> What this gives us is a function than can be invoked with a single >> parameter: >> (pf 44) >> And it can be invoked with 2 parameters: >> (pf :defined? 44) >> >> >> I keep looking at these 2 lines thinking maybe they reveal how >> different parameters cause a different action to be triggered, but I >> am unable to make sense of it: >> >> `(fn ([x#] (match [x#] ~@body)) >> ([x# y#] >> >> The anonymous fn is simply being returned? Is it returned at compile >> time, or run time? >> >> >> >> -- >> -- >> 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. >> >> >> > > > -- > Ben Wolfson > "Human kind has used its intelligence to vary the flavour of drinks, which > may be sweet, aromatic, fermented or spirit-based. ... Family and social > life also offer numerous other occasions to consume drinks for pleasure." > [Larousse, "Drink" entry] > > -- > -- > 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. > > > -- Telegram, Simply Beautiful CMS https://telegr.am Lift, the simply functional web framework http://liftweb.net Follow me: http://twitter.com/dpp Blog: http://goodstuff.im -- -- 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.
