From: Pat Maddox <devlists-ruby-talk@...>
Date: 2006-08-07T10:21:16+09:00
Subject: Re: Singleton methods with method_missing


On Monday, August 07, 2006, at 4:08 AM, Sander Land wrote:
>On 8/6/06, Pat Maddox <devlists-ruby-talk@devlists.com> wrote:
>> I don't have any practical use for this, but I was experimenting a bit.
>> I just want to create singleton methods on objects of this class.
>> There's more to come, but this is the part I'm stuck at :)
>>
>> This code doesn't actually work, but I think it's close (I hope?)
>>
>> When I do this I get "NameError: undefined local variable or method `m'
>> for #<Class:#<OneShot:0x32cf74>>" so I think it means that m just isn't
>> in scope for the singleton class.  How can I define the singleton
>> method?  Is there any way to use binding here?
>
>It is close :)
>class<<self .... end is not a closure (only blocks are), so m is not
>defined there. A solution is to use blocks for everything, using *eval
>functions.
>
>class OneShot
> def method_missing(m,*a)
>   puts "method #{m} missing!"
>   (class<<self;self;end).class_eval {
>     define_method(m) { puts m.to_s }
>   }
>   send(m)
> end
>end
>
>a = OneShot.new
>a.foo
>a.foo
>b = OneShot.new
>b.foo
>b.foo
>
>output:
>method foo missing!
>foo
>foo
>method foo missing!
>foo
>foo
>
>Which is what you want, right?
>


Okay yeah, that works great.  Why do you use class_eval there?  fwiw, I  
tried it with instance_eval and got the same behavior.  I still don't  
really understand the difference.

Alright, so now that that's out of the way, I want to be able to  
undefine methods.  I also want to keep track of any that get undefined  
so that they can't be redefined at all:

class OneShot
  def initialize
    @methods = []
  end

  def method_missing(m, *args, &block)
    puts @methods.inspect
    raise "Sorry bud, you already had your shot" if @methods.include?(m)
    puts "Defining #{m}"
    (class << self; self; end).class_eval { define_method(m) { puts  
m.to_s } }
    send(m)
  end

  private
  def remove_method(m)
    @methods << m
    (class << self; self; end).class_eval { remove_method(m) }
  end
end

If I make remove_method public, I can just call o.remove_method(:foo)  
and it works as expected.  Apparently remove_method is a private method  
though, so I was wondering if I could keep it that way.  The code I saw  
in Ruby Cookbook showed something like
class << o
  remove_method :foo
end

That doesn't actually call my custom remove_method though.  If I  
understand correctly, it's because that syntax is actually calling  
remove_method on the singleton class.  Other than making remove_method  
public, I'm not sure how to do what I want.

Pat
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