From: 7stud -- <bbxx789_05ss@...>
Date: 2011-04-17T03:20:50+09:00
Subject: Re: looking for an "inversion" pattern

Kevin Mahler wrote in post #992945:
> Fearless Fool wrote in post #992929:
>> I'd like to create a method +foo+ that transforms:
>>
>>    my_obj.foo.some_method(*args)
>>
>> to
>>
>>   MyClass.some_method(my_obj, *args)
>
>   class MyClass
>     def self.some_method(obj, *args)
>       puts "MyClass.some_method:"
>       puts "obj: #{obj.inspect}"
>       puts "args: #{args.inspect}"
>     end
>   end
>
>   def define_foo(obj, clazz, method)
>     obj.singleton_class.class_eval do
>       define_method :foo do
>         Class.new do
>           define_method method do |*args|
>             clazz.send(method, obj, *args)
>           end
>         end.new
>       end
>     end
>   end
>
>   my_obj = "my_obj thing"
>   args = [1,2,3]
>   define_foo(my_obj, MyClass, :some_method)
>   my_obj.foo.some_method(*args)
>
>   # =>
>   # MyClass.some_method:
>   # obj: "my_obj thing"
>   # args: [1, 2, 3]
>
> Returning facade or proxy objects like this can be an elegant solution
> to certain problems. It's rather high on the abstraction ladder, though,
> and without knowing the context I would wonder if more direct solutions
> are possible.

I don't know what's conceptually easier to understand, but in ruby there 
is no limit to how high you can stack singleton methods, e.g.:

obj.foo
obj.foo.some_method

If obj.foo() returns obj's singleton class, then some_method() is being 
called as a class method of the singleton class.  That means some_method 
is a method inside the singleton class's singleton class:

obj's class
   ^
   |
   |
singleton2: some_method()
   ^
   |
   |
singleton1: foo() -> returns singleton1
   ^
   |
   |
  obj


obj.foo.some_method

And you can add as many singleton classes to the method lookup path as 
you want.  For instance, if you have this call:

obj.foo.some_method.do_stuff

and obj.foo.some_method() returns singleton2, then do_stuff() is being 
called as a class method of singleton2, i.e. do_stuff() is a method in a 
parent class, singleton3, above singleton2.

So in the original example at the top of the post, if the 
creating-a-new-anonymous-class-with-a-method-named-some_method-and-returning-an-instance-of-that-class 
is too hard to understand, you could do this:

class MyClass
    def self.some_method(obj, *args)
      puts "MyClass.some_method:"
      puts "obj: #{obj.inspect}"
      puts "args: #{args.inspect}"
    end
  end

  def define_foo(obj, clazz, method)
    obj.singleton_class.class_eval do
      singleton = self

      define_method :foo do
        #Class.new do
          #define_method method do |*args|
            #clazz.send(method, obj, *args)
          #end
        #end.new


        #In here, self is equal to the obj that
        #will eventually call foo()--not obj.singleton_class.
        #Hence, the need to do singleton = self above.

        singleton.singleton_class.class_eval do
          define_method method do |*args|
            clazz.send(method, obj, *args)
          end
        end

        singleton #return the singleton class
      end

    end

  end

  my_obj = "my_obj thing"
  args = [1,2,3]
  define_foo(my_obj, MyClass, :some_method)
  my_obj.foo.some_method(*args)

--output:--
MyClass.some_method:
obj: "my_obj thing"
args: [1, 2, 3]

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