It's also not as maintainable as using a single multi-method. Like I said, not pretty, but it works.
Paul On Fri, Mar 20, 2009 at 11:38 AM, David Nolen <dnolen.li...@gmail.com>wrote: > 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 -~----------~----~----~----~------~----~------~--~---
