From: dblack@...
Date: 2007-02-08T21:18:40+09:00
Subject: Re: access to constants from instance_eval

Hi --

On Thu, 8 Feb 2007, David Chelimsky wrote:

> Hi all,
>
> It seems that Ruby won't allow me to directly access class constants
> from instance_eval:
>
> irb(main):001:0> class Foo
> irb(main):002:1>   BAR = "bar"
> irb(main):003:1>   def bar
> irb(main):004:2>     BAR
> irb(main):005:2>   end
> irb(main):006:1> end
> => nil
> irb(main):007:0> foo = Foo.new
> => #<Foo:0x10d97f0>
> irb(main):008:0> foo.class.constants
> => ["BAR"]
> irb(main):009:0> foo.bar
> => "bar"
> irb(main):010:0> foo.instance_eval { bar }
> => "bar"
> irb(main):011:0> foo.instance_eval { BAR }
> NameError: uninitialized constant BAR
>       from (irb):11
>       from (irb):11:in `instance_eval'
>       from (irb):11
>
> Can anybody explain to me why this might be the case (is there a
> philosophical reason that this is correct?) and/or some meta-magic
> spell I can use to coerce instance_eval into recognizing foo's
> constants?

foo doesn't have any constants; only Foo does.  instance_eval changes
self, but it's still in the same scope.  So the constants available
inside the block are the same as those available outside the block.

(You can see foo's instance variables, if any, inside the block, but
that's because the visibility of instance variables depends on
'self'.)


David

-- 
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