This works well as long as you don't mind the perf hit for the second dispatch :)
On Fri, Mar 20, 2009 at 11:35 AM, Paul Stadig <p...@stadig.name> wrote: > You could use multiple multi-methods: > > user=> (defmulti plus-int (fn [x y] (type y))) > #'user/plus-int > user=> (defmethod plus-int :default [x y] (println "the first is an int")) > #<MultiFn clojure.lang.mult...@11992cc> > user=> (defmethod plus-int java.lang.Double [x y] (println "one of each")) > #<MultiFn clojure.lang.mult...@11992cc> > user=> (defmulti plus (fn [x y] (type x))) > #'user/plus > user=> (defmethod plus java.lang.Integer [x y] (plus-int x y)) > #<MultiFn clojure.lang.mult...@40f603> > user=> (plus 1 5) > the first is an int > nil > user=> (plus 1 3.0) > one of each > nil > user=> > > Not very pretty. (Written backwards courtesy of the REPL ;)) > > > Paul > > > On Fri, Mar 20, 2009 at 11:18 AM, Rich Hickey <richhic...@gmail.com>wrote: > >> >> >> >> On Mar 20, 10:56 am, Konrad Hinsen <konrad.hin...@laposte.net> wrote: >> > Providing a :default implementation for multimethods is a very common >> > and useful technique, but it is really useful only for multimethods >> > that dispatch on a single argument. >> >> I disagree about that. No dispatch value, composite or not, is still a >> valid concept. >> >> > What I am looking for is an >> > equivalent technique for multiple-argument dispatch. >> > >> > Suppose you have a multimethod + of two arguments, and you want to >> > dispatch on both of them: >> > >> > (defmulti + (fn [x y] [(type x) (type y)])) >> > >> > You can then write implementations such as >> > >> > (defmethod + [java.lang.Integer java.lang.Double] ...) >> > >> > You can also provide a default implementation, of course: >> > >> > (defmethod + :default ...) >> > >> > But suppose you want to provide a default for one argument only? >> > Something like >> > >> > (defmethod + [java.lang.Integer ::any] ...) >> > >> >> I think it is best to think about this differently than :default, it's >> more about a universal parent than about a missing dispatch value. >> >> > i.e. a multimethod that matches all invocations in which the first >> > argument is an integer. I don't currently see a simple way to do >> > this. For types in the Java class hierarchy, you can use Object as >> > the parent of all types, but there is nothing equivalent in Clojure's >> > ad-hoc hierarchies. >> > >> > Would it be a good idea to provide the possiblity to add a universal >> > parent to hierarchies? Or would that create any problems? Is there >> > another solution for the situation I described? >> > >> >> Yes, don't know, and no. I briefly looked at this but only got as far >> as to decide Object couldn't be the universal parent. I think you have >> to reserve a value that will never otherwise be used. >> >> Rich >> >> >> > > > > --~--~---------~--~----~------------~-------~--~----~ 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 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 -~----------~----~----~----~------~----~------~--~---
