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From: Andrew Berkeley <andrew.berkeley.is@googlemail.com>
Newsgroups: comp.lang.ruby
Subject: class_eval doesn't find const_missing
Date: Thu, 21 Apr 2011 14:20:46 -0500
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.ruby:3332

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

-- 
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