From: "David A. Black" <dblack@...>
Date: 2008-03-22T02:37:42+09:00
Subject: Re: dynamically defining a method on an instance that references vars in enclosing scope?

Hi --

On Sat, 22 Mar 2008, mutahhir hayat wrote:

>>  I know that the problem is scope. That's why I suggested a way to
>>  manipulate the scope :-)
>>
>>  I definitely would not (and deliberately did not) throw a global
>>  variable at it. Also, defining x inside MyClass doesn't help, because
>>  that's also a different local scope.
>
> I wasn't sure, but running an intermix of your code with the original
> still would give the same error in irb. I wasn't advocating the use of
> globals, ( though, come to think about it, i do feel bad now even
> mentioning it :P) but I didn't consider that x wouldn't be usable.

I don't see the error:

irb(main):001:0>   class Object
irb(main):002:1>     def singleton_class
irb(main):003:2>       class << self
irb(main):004:3>         self
irb(main):005:3>       end
irb(main):006:2>     end
irb(main):007:1>   end
=> nil
irb(main):008:0> 
irb(main):009:0*   instance = Object.new
=> #<Object:0x64f50>
irb(main):010:0>   x = 1
=> 1
irb(main):011:0> 
irb(main):012:0*   instance.singleton_class.class_eval {
irb(main):013:1*     define_method("foo") { puts x }
irb(main):014:1>   }
=> #<Proc:0x00058728@(irb):13>
irb(main):015:0> 
irb(main):016:0*   instance.foo
1

> :) I'm still getting used to this metaprogramming thing, but how did
> you change scope? In my view, the scope remains exactly same, you just
> allowed the method to be inserted into the object instance
> dynamically.

That's the thing: I didn't change scope. The block that uses x (line
013 above) is in the same scope as the line that defines x (010) -- at
least, in the same scope block-style. (Meaning: the block can see all
the locals that are already in scope.)

> Anyways, simplest i found was: (swearing never to mention global again... :P)
>
> class MyClass
>   attr_accessor :x
>   #... or if you want you could do
>   # def initialize(x); @x = x; end
> end
>
> instance = MyClass.new
>
> def instance.foo
> puts x
> end
>
> instance.x = 1
> instance.foo
>
>
> would work.

Yes, but you've shifted to instance variables. That's a more common
use case than the class_eval/define_method one, but they're both worth
knowing about.


David

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