From: Stephen Prater <stephenp@...>
Date: 2011-04-22T06:34:21+09:00
Subject: Re: class_eval doesn't find const_missing

http://stackoverflow.com/questions/3015947/ruby-how-does-constant-lookup-work-in-instance-eval-class-eval

Const lookup varies between 1.8, 1.9.1, and 1.9.2.

Basically, constants are lexically scoped.

You can use "const_get" I believe to lookup your constant in the  
current binding rather than in the lexical scope.

stephen

On Apr 21, 2011, at 2:20 PM, Andrew Berkeley wrote:

> I am trying to write a DSL which uses method_missing and const_missing
> to catch upper and lowercase names (specified in isolation) and fetch
> the asscoated (already initialized) objects. This works great until I
> need to use class_eval which seemingly doesn't follow the custom
> const_missing method.
>
> Here's a more simple example of a class with a constant and
> #method_missing and #const_missing methods.
>
> class DummyClass
>
>  NAME = 'dummy constant'
>
>  def self.method_missing(method,*args,&block)
>    return "this #{method} was captured by #method_missing"
>  end
>
>  def self.const_missing(constant)
>    return "this #{constant} was captured by #const_missing"
>  end
>
> end
>
> Accessing the constant is no problem
>> DummyClass::NAME
> => "dummy constant"
>
> Unknown methods and constants are handled as expected
>> DummyClass::ANOTHER_CONSTANT
> => "this ANOTHER_CONSTANT was captured by #const_missing"
>> DummyClass.some_method
> => "this some_method was captured by #method_missing"
>
> Accessing the constant via class_eval is variable - it works when
> evaluating strings but not blocks.
>> DummyClass.class_eval "NAME"
> => "dummy constant"
>> DummyClass.class_eval {NAME}
> NameError: uninitialized constant NAME
>  from (irb):14
>  from (irb):14:in `class_eval'
>  from (irb):14
>  from :0
>
> This is partly understood since blocks are scoped to the context in
> which they were created. But even if the explicit constant is not
> recognised, why isn't the class #const_missing method invoked?
>
> Does the class_eval method allow the method_missing method to be used?
>> DummyClass.class_eval "dummy_method"
> => "this dummy_method was captured by #method_missing"
>> DummyClass.class_eval {dummy_method}
> => "this dummy_method was captured by #method_missing"
>
> Yes. #method_missing works as expected, in both string and block
> formats. So only #const_missing is causing a problem.
>
> Interestingly, an explicit call to #const_missing inside a class_eval
> block DOES work.
>> DummyClass.class_eval {const_missing :ANOTHER_CONSTANT}
> => "this ANOTHER_CONSTANT was captured by #const_missing"
>
> Anybody understand this, and is there a way to get around it?
>
> Cheers
>
> -- 
> Posted via http://www.ruby-forum.com/.
>
>