[ruby-talk:11492] Re: eval-context problem (bug?)

Hello --

On Sun, 25 Feb 2001, Clemens Wyss wrote:

> Hi all,
> the following code (part of this is from "Programming Ruby"):

[...]

> Produces the following error:
> D:\Projekte\rubychannel\ProgrammingRuby\html\test.rb:34:in `eval':
> (eval):15:in `getBinding': undefined method `doc' for Test::Example:Class
> (NameError) 
>         from D:\Projekte\rubychannel\ProgrammingRuby\html\test.rb:34
> 
> This does not happen when eval-uating in the default-(i.e. main-)context.
> 
> Is this a bug or an executing context related feature?

I believe it's all feature.  Here's a stripped-down little test:

  content = '
  class Module
    p self

    def doc(s)
      puts s
    end
  end

  class Example
    doc("Doc...")
  end
  '

  class Test
    def getBinding
      binding
    end
  end

  eval(content, Test.new.getBinding)

Output:

  Test::Module
  wyss.rb:21:in `eval': (eval):11:in `getBinding': undefined method
  `doc' for Test::Example:Class (NameError) from wyss.rb:21

The eval is bound to the context of class Test -- so when the string
gets eval'd, the class you're modifying is class Test::Module, not of
class Module.  

This becomes visible through several ways of forcing it to work.
(All of these are geared toward my simplified example -- I haven't
fitted them to yours, but I think the principles are the same.)

1. Rewrite class Example:

  class Example
    Test::Module.new.doc("Doc...")
  end

Output:
  Test::Module
  Doc...

2. Rewrite class Test:

  class Test
    def getBinding
      Object.module_eval "binding"
    end
  end

Output:

  Module
  Doc...

3. Do the one-argument test first:

  eval(content)
  eval(content, Test.new.getBinding)

Output:
  Module
  Doc...
  Test::Module
  Doc...


None of these methods (except #2, maybe?) are very sound or practical,
but they show what (I think) is going on.


David

-- 
David Alan Black
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