From: Rick DeNatale <rick.denatale@...>
Date: 2010-04-30T09:18:20+09:00
Subject: Re: Hijacking `super'

On Thu, Apr 29, 2010 at 4:19 PM, Michael Jackson <mjijackson@gmail.com> wrote:
> I know the following is probably impossible, but I just thought I'd
> ping the list to see if any adventurous meta programmer knows of a way
> to essentially change the binding of a block. Here's essentially what
> I'm trying to do:
>
> $ cat super.rb
> class A
>  def call
>    "A"
>  end
> end
>
> class B < A
>  def call(&b)
>    instance_eval(&b)
>  end
> end
>
> puts B.new.call { super }
>
> $ ruby super.rb
> super.rb:13:in `block in <main>': super called outside of method (NoMethodError)
>        from super.rb:9:in `instance_eval'
>        from super.rb:9:in `call'
>        from super.rb:13:in `<main>'
>
> Now, when `super' is used inside the block it tries to call a method
> with the same name as the method that it's used in, but in the parent
> class. Obviously, since the block is created outside of any method
> here Ruby complains that super was called outside of a method.
> However, if you change the code to use a string and plain old `eval',
> you can mimic the desired behavior.
>
> $ cat super1.rb
> class A
>  def call(s)
>    "A"
>  end
> end
>
> class B < A
>  def call(s)
>    eval(s)
>  end
> end
>
> puts B.new.call 'super'
>
> $ ruby super1.rb
> A
>
> The only problem now is that you're limited to using strings and you
> can't use blocks anymore. The conclusion I've come to is that you
> essentially need to be able to alter the binding of the given block
> within `call' to be the binding that exists within the `call' method.
> Then, super should be able to find the method that it's called in and
> the superclass. However, unlike most other things in Ruby, bindings
> are pretty opaque.
>
> Any ideas are very much appreciated.


I don't think it's a question of binding.  To evaluate super, the ruby
vm looks in the current stack frame for the method name and arguments
(if no arguments are provided on the super call).  The block
evaluation happens in it's own stack frame.  Even if the vm were to
look at calling frames for one which had a method, it would find
instance_eval before it found call.



-- 
Rick DeNatale

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