I was bitten by this multi-method problem this week and so, coincidentally was a newbie friend of mine.
This trick works perfectly and got me (us) out of the hole. Given the recent (very worthy IMHO) marketing efforts for the REPL can we have an update on what mitigations are in place for this and other re-loading REPL issues? I looked on the page <https://dev.clojure.org/display/design/Never+Close+a+REPL> that Stu created ago and it seems nothing has been updated since 2014. Does anyone know of a solid resource where we can find a clear definition of the issues the require a REPL restart and their current / best mitigations? I have also opened a document issue on the Clojure.org web site requesting that it should have a REPL guide, which should include these gotchas. You can see more there https://github.com/clojure/clojure-site/issues/218 Ray On Saturday, September 8, 2012 at 10:03:40 AM UTC+2, Denis Labaye wrote: > > > > On Wed, Sep 5, 2012 at 12:31 AM, Brian Marick <mar...@exampler.com > <javascript:>> wrote: > >> I'm trying to write exercises for multimethods. Book readers will be >> working at the repl. Multimethods are stateful in a bad way, as shown >> below. Is there some sort of trick to using multimethods at the repl, or >> should I just give up on exercises using them? >> >> ;; Two types: >> user=> (defn ship [name] (with-meta {:name name} {:type :ship})) >> user=> (defn asteroid [name] (with-meta {:name name} {:type :asteroid})) >> >> ;; The dispatch function and defmulti >> >> user=> (def classify-colliding-things >> (fn [thing1 thing2] >> [(type thing1) (type thing2)])) >> user=> (defmulti collide classify-colliding-things) >> >> ;; Actually, since the arguments can come in any order, it'd be better to >> sort the types: >> >> user=> (def classify-colliding-things >> (fn [thing1 thing2] >> (sort [(type thing1) (type thing2)]))) >> >> ;; And let's redefine the multimethod to use the new comparison function. >> >> user=> (defmulti collide classify-colliding-things) >> >> ;; OK, now we define the methods. >> >> user=> (defmethod collide [:asteroid :ship] >> [& things] >> "collide asteroid to ship") >> >> ;;; And use them with great confidence: >> >> user=> (collide (ship "Space Beagle") (asteroid "Malse")) >> IllegalArgumentException No method in multimethod 'collide' for dispatch >> value: [:ship :asteroid] clojure.lang.MultiFn.getFn (MultiFn.java:121) >> >> ;;; The redefinition didn't take >> > > here is a hack: define a var with the multimethod name: > > user> (def collide nil) > ; #'user/collide > user> (defmulti collide classify-colliding-things) > ; #'user/collide > user> (defmethod collide [:asteroid :ship] > [& things] > "collide asteroid to ship") > ; #<MultiFn clojure.lang.MultiFn@9fe5c5> > user> (collide (ship "Space Beagle") (asteroid "Malse")) > ; "collide asteroid to ship" > > > > When writting multimethod I always preceed their definitions like this: > > ; remove existing definition > (def mymulti nil) > (defmulti mymulti ...) > > > Also, I "heard" that this problem don't exists in nRepl (?) > > Denis > > > > > >> ----- >> Brian Marick, Artisanal Labrador >> Contract programming in Ruby and Clojure >> Occasional consulting on Agile >> Writing /Functional Programming for the Object-Oriented Programmer/: >> https://leanpub.com/fp-oo >> >> >> -- >> You received this message because you are subscribed to the Google >> Groups "Clojure" group. >> To post to this group, send email to clo...@googlegroups.com >> <javascript:> >> Note that posts from new members are moderated - please be patient with >> your first post. >> To unsubscribe from this group, send email to >> clojure+u...@googlegroups.com <javascript:> >> 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 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.
