ruby - Attempt at understanding the double-dispatch pattern -

i've been trying grok double-dispatch pattern , having hard time. attempted sample program myself understand. here's gist. decided attempt without double dispatch , solution didn't more terrible usual. doing wrong?

edit: per suggestion, i've posted question here. keeping link around redirects.

in single dispatch---what see in modern oo languages---a method dispatched based on run-time type of single object. shows dot operator (in ruby, java, javascript, etc.) or arrow operator (perl, c++).

# look, ma single dispatch! # method on obj's run-time type called dentist.work_on(patient) 

double dispatch, then, based on run-time type of two objects. there few ways look; , on object should method live?

# hmm, looks weird. # method in dentist.class or patient.class? (dentist, patient).do_dentistry()   # okay, looks more familiar; method lives on obj1.class # works in static-typed languages support double dispatch # in declare type of method parameters. dentist.work_on(patient)  class dentist    def work_on(adult patient); ...; end    def work_on(child patient); ...; end end 

languages groovy have multiple dispatch generalize second example above; consider run-time types of all parameters when choosing method run. see example this blog post groovy , multiple dispatch.

most modern oo languages have single dispatch , multiple dispatch pattern attempt benefits of multiple dispatch language. works dynamic languages ruby. works doing single dispatch twice in row. first method call call method on second object.

class dentist     def work_on(patient)        patient.dispatch_work(self)     end     def work_on_adult(patient)       drill_as_hard_as_you_can(patient)     end     def work_on_child(patient)       use_bubble_gum_toothpaste(patient)       give_toothbrush_to(patient)     end end  class doctor     def work_on(patient)        patient.dispatch_work(self)     end     def work_on_adult(patient)       do_checkup(patient)     end     def work_on_child(patient)       assure_presence_of(patient.guardian)       ask_questions_to(patient.guardian)       do_checkup(patient)       give_cheap_toy_to(patient)     end end    class adult     def dispatch_work(dentist)       dentist.work_on_adult(self)     end end  class child     def dispatch_work(dentist)       dentist.work_on_child(self)     end end 

the double dispatch pattern call low-level pattern because other patterns built on it. example, visitor pattern relies heavily on double dispatch pattern.

---

update saw gists. first gist isn't doing double dispatch. sure, you're dispatching twice, you're not changing behavior in second dispatch. change double dispatch i'd this.

class chicken   def make_dispatch dish     dish.make_with_chicken self   end end  class beef   def make_dispatch dish     dish.make_with_beef self   end end   module dish   def make meat     meat.make_dispatch self   end end  class sandwich   include dish    def make_with_chicken chicken     puts "grilled chicken sandwich"   end    def make_with_beef beef     puts "roast beef sandwich"   end end  class stew   include dish    def make_with_chicken chicken     puts "thai curry"   end    def make_with_beef beef     puts "beef stew"   end end  class casserole   include dish    def make_with_chicken chicken     puts "chicken pot pie--or something"   end    def make_with_beef beef     puts "shepard's pie"   end end  sandwich.new.make(chicken.new) stew.new.make(chicken.new) casserole.new.make(beef.new)