Asp Forum - class destruction (evil genius metaprogramming

Giles Bowkett

6/15/2007 1:14:00 AM

I want to write a module which, when included in another module,
destroys every method of a class in that module. I realize this isn't
really a very useful thing to do, but I want to do it to see if it can
be done. I want to actually include the module and have it replace all
existing methods of this class with just one method.

In Python you can change an object's class in the middle of your
program by reassigning its magical __class__ variable. That would be a
very simple way to implement this idea. Doesn't seem possible here,
though.

Here's an attempt which failed:

>> class Muppet
>> def show
>> "it's the muppet show!"
>> end
>> end
=> nil
>> kermit = Muppet.new
=> #<Muppet:0x123129c>
>> kermit.show
=> "it's the muppet show!"
>> class Muppet
>> remove_method(:show)
>> end
=> Muppet
>> kermit.show
NoMethodError: undefined method `show' for #<Muppet:0x123129c>
from (irb):36
from :0

So far so good, but I want to get rid of *every* method. Doing what I
just did programmatically, iterating over all available methods,
that's the thing which still eludes me.

None of these attempts work:

>> kermit.methods.each{|m| class << Muppet ; (remove_method(m)) ; end}
NameError: undefined local variable or method `m' for #<Class:Muppet>
from (irb):43
from (irb):43:in `each'
from (irb):43
from :0
>> kermit.methods.each{|m| class Muppet; remove_method(m.to_sym) ; end}
NameError: undefined local variable or method `m' for Muppet:Class
from (irb):44
from (irb):44:in `each'
from (irb):44
from :0
>> kermit.methods.each{|m| Muppet.class_eval(remove_method(m))}
NoMethodError: undefined method `remove_method' for #<Object:0x349f4>
from (irb):45
from (irb):45:in `each'
from (irb):45
from :0

I have to admit, it's not necessarily a bad thing if this task proves
impossible, but I feel like it *should* be possible.

--
Giles Bowkett

Blog: http://gilesbowkett.bl...
Portfolio: http://www.gilesg...

16 Answers

Rick DeNatale

6/15/2007 1:55:00 AM

On 6/14/07, Giles Bowkett <gilesb@gmail.com> wrote:
> I want to write a module which, when included in another module,
> destroys every method of a class in that module. I realize this isn't
> really a very useful thing to do, but I want to do it to see if it can
> be done. I want to actually include the module and have it replace all
> existing methods of this class with just one method.
>
> In Python you can change an object's class in the middle of your
> program by reassigning its magical __class__ variable. That would be a
> very simple way to implement this idea. Doesn't seem possible here,
> though.
>
> Here's an attempt which failed:
>
> >> class Muppet
> >> def show
> >> "it's the muppet show!"
> >> end
> >> end
> => nil
> >> kermit = Muppet.new
> => #<Muppet:0x123129c>
> >> kermit.show
> => "it's the muppet show!"
> >> class Muppet
> >> remove_method(:show)
> >> end
> => Muppet
> >> kermit.show
> NoMethodError: undefined method `show' for #<Muppet:0x123129c>
> from (irb):36
> from :0
>
> So far so good, but I want to get rid of *every* method. Doing what I
> just did programmatically, iterating over all available methods,
> that's the thing which still eludes me.
>
> None of these attempts work:
>
> >> kermit.methods.each{|m| class << Muppet ; (remove_method(m)) ; end}
> NameError: undefined local variable or method `m' for #<Class:Muppet>
> from (irb):43
> from (irb):43:in `each'
> from (irb):43
> from :0
> >> kermit.methods.each{|m| class Muppet; remove_method(m.to_sym) ; end}
> NameError: undefined local variable or method `m' for Muppet:Class
> from (irb):44
> from (irb):44:in `each'
> from (irb):44
> from :0
> >> kermit.methods.each{|m| Muppet.class_eval(remove_method(m))}
> NoMethodError: undefined method `remove_method' for #<Object:0x349f4>
> from (irb):45
> from (irb):45:in `each'
> from (irb):45
> from :0
>
> I have to admit, it's not necessarily a bad thing if this task proves
> impossible, but I feel like it *should* be possible.

kermit.class.instance_methods(false).each{|m|
Muppet.instance_eval("remove_method #{m.inspect}")}

Note 1. kermit.methods will return all methods inherited or not.
Note 2. remove method is a class method, so it's evaluated in the
context of the class object, hence Muppet.instance_eval
Note 3: you could alternatively pass symbols for the method names

kermit.class.instance_methods(false).each{|m|
Muppet.instance_eval("remove_method :#{m}")}

--
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denh...

IPMS/USA Region 12 Coordinator
http://ipmsr12.denh...

Visit the Project Mercury Wiki Site
http://www.mercuryspace...

Giles Bowkett

6/15/2007 2:27:00 AM

> kermit.class.instance_methods(false).each{|m|
> Muppet.instance_eval("remove_method #{m.inspect}")}
>
> Note 1. kermit.methods will return all methods inherited or not.
> Note 2. remove method is a class method, so it's evaluated in the
> context of the class object, hence Muppet.instance_eval
> Note 3: you could alternatively pass symbols for the method names
>
> kermit.class.instance_methods(false).each{|m|
> Muppet.instance_eval("remove_method :#{m}")}

Muahahaha! That was awesome! Soon the world will quake in fear when I
put this information to use!

But why doesn't this work?

>> kermit.methods(false).each do |m|
?> Object.instance_eval("remove_method :#{m}")
>> end
=> []
>> kermit.methods
=> 53

I tried it with Kernel also and no dice.

Tried it with something else, though, and got a great error message:

(eval):1: warning: removing `initialize' may cause serious problem

Muahahaha!

--
Giles Bowkett

Blog: http://gilesbowkett.bl...
Portfolio: http://www.gilesg...

dblack

6/15/2007 11:20:00 AM

Robert Dober

6/15/2007 11:26:00 AM

On 6/15/07, dblack@wobblini.net <dblack@wobblini.net> wrote:
> Hi --
>
> On Fri, 15 Jun 2007, Rick DeNatale wrote:
>
> > kermit.class.instance_methods(false).each{|m|
> > Muppet.instance_eval("remove_method #{m.inspect}")}
> >
> > Note 1. kermit.methods will return all methods inherited or not.
> > Note 2. remove method is a class method, so it's evaluated in the
> > context of the class object, hence Muppet.instance_eval
> > Note 3: you could alternatively pass symbols for the method names
> >
> > kermit.class.instance_methods(false).each{|m|
> > Muppet.instance_eval("remove_method :#{m}")}
>
> You can also just use the block form of instance_eval:
>
> Muppet.instance_eval { remove_method(m) }

or Muppet.send :remove_method, :m

Cheers
Robert

--
You see things; and you say Why?
But I dream things that never were; and I say Why not?
-- George Bernard Shaw

dblack

6/15/2007 1:50:00 PM

dblack

6/15/2007 1:56:00 PM

Robert Dober

6/15/2007 2:05:00 PM

On 6/15/07, dblack@wobblini.net <dblack@wobblini.net> wrote:
> Hi --
>
> On Fri, 15 Jun 2007, Robert Dober wrote:
>
> > On 6/15/07, dblack@wobblini.net <dblack@wobblini.net> wrote:
> >> Hi --
> >>
> >> On Fri, 15 Jun 2007, Rick DeNatale wrote:
> >>
> >> > kermit.class.instance_methods(false).each{|m|
> >> > Muppet.instance_eval("remove_method #{m.inspect}")}
> >> >
> >> > Note 1. kermit.methods will return all methods inherited or not.
> >> > Note 2. remove method is a class method, so it's evaluated in the
> >> > context of the class object, hence Muppet.instance_eval
> >> > Note 3: you could alternatively pass symbols for the method names
> >> >
> >> > kermit.class.instance_methods(false).each{|m|
> >> > Muppet.instance_eval("remove_method :#{m}")}
> >>
> >> You can also just use the block form of instance_eval:
> >>
> >> Muppet.instance_eval { remove_method(m) }
> >
> > or Muppet.send :remove_method, :m
>
> If you have a method called "m" :-)
Ah I see :(, Probably 50% of my errors come from spurious ":" ARRRRG

Muppet.send :remove_method, m ### TESTED ;)

Sorry.
Robert

--
You see things; and you say Why?
But I dream things that never were; and I say Why not?
-- George Bernard Shaw

Giles Bowkett

6/15/2007 8:40:00 PM

> > Why should this inspire fear?
>
> Code injection attack to own a RoR site possibly? That would be my guess.

Sort of. I'm trying to use it to build a Rails plugin called
acts_as_fox, which overrides every method on a model to return "chunky
bacon!" (It's not really that terrifying, I just kind of had a Dr.
Frankenstein moment, drunk on power type thing.)

Unfortunately, applying Rick's code directly to an ActiveRecord model
doesn't quite accomplish this, because it's missing the superclass
methods, but applying it to ActiveRecord::Base doesn't work either. I
did get it to work by doing it twice, both on the actual model and on
ActiveRecord::Base itself, but that's very unsatisfying, because I
solved the problem by cutting and pasting. (I think I understand why
it worked; ActiveRecord::Base attaches a lot of methods to its
subclasses, instead of having them inherited directly.) It also fails
to really accomplish what I want to do, because it means that making
one model acts_as_fox destroys all the other models. (I also need to
attach a method_missing to return "chunky bacon!" but that part's
obviously trivial.)

Really the quickest way to accomplish this would be to simply pop the
model out of its inheritance hierarchy - redefine the model not to
have any superclass except Object - but I don't know if that's
possible. Trying it in the most obvious way (class Foo < Object; end,
where Foo was already defined Foo < ActiveRecord::Base) results in a
TypeError with the message "superclass mismatch."

But there must be a clean way to open up the class, grab all its
methods, including those derived from superclasses, and simply
reassign them. Something like

Foo.instance_methods(true).each{|m| Foo.instance_eval("alias
:chunky_bacon " + m)}

--
Giles Bowkett

Blog: http://gilesbowkett.bl...
Portfolio: http://www.gilesg...

dblack

6/16/2007 10:31:00 AM

Devin Mullins

6/16/2007 4:05:00 PM

dblack@wobblini.net wrote:
> I do sometimes wonder what would happen if the ancestry array were
> writeable. It could be interesting. I haven't thought through the
> possible pitfalls.

You mean, like Object#become? :)

Seriously, how would that play with the core classes, which don't keep
their state in instance variables?

Devin

comp.lang.ruby

class destruction (evil genius metaprogramming

Giles Bowkett

Rick DeNatale

Giles Bowkett

dblack

Robert Dober

dblack

dblack

Robert Dober

Giles Bowkett

dblack

Devin Mullins

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