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 -~----------~----~----~----~------~----~------~--~---
