From: "David A. Black" <dblack@...>
Date: 2009-09-10T21:49:52+09:00
Subject: Re: defining a singleton method dynamically

Hi --

On Thu, 10 Sep 2009, Nilesh Trivedi wrote:

> How do I define a singleton method dynamically?
>
> I have code similar to this:
>
> def foo(obj,label)
>  def obj.name; label; end #we want to define a singleton method called
> "name" on obj which returns the value in label
>  puts obj.name #this is an example. Actually I have some code here which
> expects this singleton method in obj
> end
>
> When I try to run this as foo(Object.new,"xyz"), it throws a "NameError:
> undefined local variable or method `label' for #<Object:0xb7b02018>" whereas
> I want it to print "xyz" to screen.
>
> Of course, line 2 in method foo is not correct. But what is the correct way?

def starts a new local scope, so it won't see label. You need to get
into obj's singleton class, using code blocks rather than class and
def blocks:

   def foo(obj, label)
     (class << obj; self; end).class_eval do
       define_method("name") { label }
     end
   end

You could also do something with eval but I prefer to use the code
block approach rather than the string-eval'ing approach.

(Still hoping for Object#singleton_class to make the access to the
singleton class a little less awkward :-)


David

-- 
David A. Black / Ruby Power and Light, LLC / http://www.rubypal.com
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