Hi guys, I am working through the pre-release second edition of Joy of Clojure's section on multi-methods (section 9.2.~ : pg. 313), and am getting different outputs from what they have printed in the book. I could just skip over it, but I really want to understand this stuff. Could someone tell me how to get the *(compile-cmd osx) *call at the end of the code to produce the expected output: *"/usr/bin/gcc" *? I have commented out redundancy and moved a couple of lines for readability. Thanks in advance!
J > (ns joy.udp > (:refer-clojure :exclude [get])) > > (defn beget [this proto] > (assoc this ::prototype proto)) > > (defn get [m k] > (when m > (if-let [[_ v] (find m k)] > v > (recur (::prototype m) k)))) > > (def put assoc) > > ;;;;;;; compiler > (defmulti compiler :os) > (defmethod compiler ::unix [m] (get m :c-compiler)) > (defmethod compiler ::osx [m] (get m :llvm-compiler)) > > (def clone (partial beget {})) > > (def unix {:os ::unix, :c-compiler "cc", :home "/home", :dev "/dev"}) > > (def osx (-> (clone unix) > (put :os ::osx) > (put :llvm-compiler "clang") > (put :home "/Users"))) > > ;;;;;;; home > (defmulti home :os) > (defmethod home ::unix [m] (get m :home)) > (defmethod home ::bsd [m] "/home") > > ;; the error on the call to (home osx) is contingent upon toggling the > following lines. ;(derive ::osx ::unix) > ;(derive ::osx ::bsd) > (prefer-method home ::unix ::bsd) > ;(remove-method home ::bsd) > (derive (make-hierarchy) ::osx ::unix) > > ;;;;;;; compile-cmd > (defmulti compile-cmd (juxt :os compiler)) > > (defmethod compile-cmd [::osx "gcc"] [m] > (str "/usr/bin/" (get m :c-compiler))) > > (defmethod compile-cmd :default [m] > (str "Unsure where to locate " (get m :c-compiler))) > ;;;;;;;;;;;;;;;;;;;;;; > > (home osx) > ;=> java.lang.IllegalArgumentException: No method in multimethod 'home' for > dispatch value: :joy.udp/osx … > ;; Should be: ;=> "/Users" (compile-cmd osx) ;=> "Unsure where to locate cc" ;; Should be: ;=> "/usr/bin/gcc" > (compile-cmd unix) ;=> "Unsure where to locate cc" ;; this is the expected output -- 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/d/optout.