From: 7stud -- <bbxx789_05ss@...>
Date: 2009-09-18T03:11:45+09:00
Subject: Re: Mucking about with dynamically adding methods to objects

Paul Smith wrote:
> 2009/9/17 Jes�s Gabriel y Gal�n <jgabrielygalan@gmail.com>:
>>> end
>> irb(main):013:1> def talkify(obj,str)
>> irb(main):023:0> A.new.talkify o,"hi"
>> => #<Proc:0xb7d64f40@(irb):15>
>> irb(main):024:0> o.talk
>> hi
>> => nil
>>
>> We call define_method in the singleton class of obj. We need the
>> instance_eval because define_method is private.
>>
>> Hope this helps,
>>
> 
> Thanks
> 
> I think I have a lot more reading to do before
> 
> irb(main):014:2> (class << obj;self;end).instance_eval do
> irb(main):015:3* define_method :talk do
> 
> makes any kind of sense.
>

This isn't going to work:

def talkify(obj, str)
  def obj.talk
    puts str
  end
end

because in ruby nested methods do not form "closures".  That means a 
nested method cannot see the local variables(including the parameter 
variables) in the enclosing method.

However, blocks can see variables in the surrounding scope, for example:

def meth
  x = 10
  arr = [1, 2, 3]

  arr.each{|num| puts num + x}
end

--output:--
11
12
13

So that suggests a strategy for devising a solution to your problem: 
try to employ a block to capture the variables in the surrounding scope 
instead of a method definition.

This is what I came up with:

class FairyGodmother


  def get_singleton_class(obj)

    class << obj
      self
    end

  end


end


1) Inside a class definition, like:

class << obj
  self
end

and outside any method definitions, self is the class object, which in 
this case is the singleton class of obj.


2) A class definition actually returns the last statement evaluated 
inside the class, for instance:

return_val = class A
  10
end

p return_val

--output:--
10

3) A method returns the value of the last statement evaluated in the 
method.

The method get_singleton_class(obj) returns the singelton class of obj 
because this:

  def get_singleton_class(obj)

    class << obj
      self
    end

  end

becomes this:

  def get_singleton_class(obj)

    obj_singleton

  end

which returns obj_singleton.  (See p. 392 in your book if that's not 
clear.)


Then you can define talkify() like this:

class FairyGodmother

  def talkify(obj, str)
    obj_singleton = get_singleton_class(obj)
    p = Proc.new {puts str}
    obj_singleton.send(:define_method, "talk", p)
  end

end

fairy = FairyGodmother.new()
pinochio = Object.new()

fairy.talkify(pinochio, "hello")
pinochio.talk


--output:--
hello

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