multimethod

I read many article about how to implement multimethod in c++: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1529.html http://www.codeproject.com/KB/recipes/mmcppfcs.aspx http://lambda-the-ultimate.org/node/2590 http://parasol.tamu.edu/people/peterp/omm/ why there are no multimethod in c++? why do not they get supported by c++ standard? Straight from the horse's mouth: "I rejected multi-methods with regret, because I liked the idea, but couldn't find an acceptable form under which to accept it." Bjarne Stroustrup, The Design and Evolution of C++ (p297) And later: "Multi-methods is

Consider this example where a subclass has a multi method with no signature and one with a slurpy parameter: class Foo { multi method do-it { put "Default" } multi method do-it ( Int $n ) { put "Int method" } multi method do-it ( Str $s ) { put "Str method" } multi method do-it ( Rat $r ) { put "Rat method" } } class Bar is Foo { multi method do-it { put "Bar method" } multi method do-it (*@a) { put "Bar slurpy method" } } Foo.new.do-it: 1; Foo.new.do-it: 'Perl 6'; Foo.new.do-it: <1/137>; Foo.new.do-it; put '-' x 10; Bar.new.do-it: 1; Bar.new.do-it: 'Perl 6'; Bar.new.do-it: <1/137>; Bar.new.do

I'm using multimethods to parse command line commands and their arguments. (defmulti run (fn [command args] command)) (defmethod run :default [& _] ...) ^{:args "[command]"} (defmethod run "help" [_ & [args]] "Display command list or help for a given command" ...) ^{:args ""} (defmethod run "version" [_ & [args]] "Print program's version" ...) (defn -main [& args] (run (first args) (next args))) When I try to access the metadata, for a specific method, clojure returns nil : (meta ((methods run) "help")) There's no such possibility. The first reason (straightforward one) is that defmethod doesn

In Clojure 1.5.0, how can I provide a custom pretty-printer for my own record type, defined with defrecord. (defrecord MyRecord [a b]) (defmethod print-method MyRecord [x ^java.io.Writer writer] (print-method (:a x) writer)) (defmethod print-dup MyRecord [x ^java.io.Writer writer] (print-dup (:a x) writer)) (println (MyRecord. 'a 'b)) ;; a -- OK (clojure.pprint/pprint (MyRecord. 'a 'b)) ;; {:a a, :b b} -- not OK, I want a I would like clojure.pprint/pprint to also use my cutsom printer (which now, should just pretty-prints whatever is in the field a of the record for illustration purposes).

More broadly this question is about various approaches to the expression problem . The idea is that your program is a combination of a datatype and operations over it. We want to be able to add new cases without recompiling the old classes. Now Haskell has some really awesome solutions to the expression problem with the TypeClass . In particular - we can do: class Eq a where (==) :: a -> a -> Bool (/=) :: a -> a -> Bool member :: (Eq a) => a -> [a] -> Bool member y [] = False member y (x:xs) = (x == y) || member y xs Now in Clojure there are multimethods - so you can do: (defmulti area :Shape)

I'm a Clojure novice and was looking for some concrete examples of when to use protocols and when to use multimethods. I know that protocols are generally geared towards creating a type hierarchy and typical OOP things, that they were added to the language after multimethods, and that protocols generally have better performance so my question is this: Are protocols meant to replace multimethods? If not, could you give me an example where I would use multimethods instead of protocols? Arthur Ulfeldt Multimethods are more powerful and more expensive, use protocols when they are sufficient but if