Access to constants from instance_eval

ruby-talk
1

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?

Thanks,
David

2

Hi --

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

···

On Thu, 8 Feb 2007, David Chelimsky wrote:

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A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black\)
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A. Ruby Power and Light, LLC (http://www.rubypal.com)

3

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?

I doubt this is what you are after, but:

>> class Foo
>> BAR = "bar"
>> def bar
>> BAR
>> end
>> end
=> nil
>> foo = Foo.new
=> #<Foo:0x11004cc>
>> foo.instance_eval { self.class.const_get(:BAR) }
=> "bar"

James Edward Gray II

···

On Feb 8, 2007, at 5:46 AM, David Chelimsky wrote:

4

Hi,

>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
[...]
>irb(main):011:0> foo.instance_eval { BAR }
>NameError: uninitialized constant BAR

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.

`Scope' is not neccessarily "self":

  irb(main):001:0> class C ; X = "i" ; end
  => "i"
  irb(main):002:0> X = "o"
  => "o"
  irb(main):003:0> C.instance_eval { X }
  => "o"
  irb(main):004:0> C.class_eval { X }
  => "o"
  irb(main):005:0> C.instance_eval { self::X }
  => "i"
  irb(main):006:0> C.class_eval { self::X }
  => "i"
  irb(main):007:0>

Is there a documentation that explains the differences between "scope",
"context", "self" and whatelse?

Bertram

···

Am Donnerstag, 08. Feb 2007, 21:18:40 +0900 schrieb dblack@wobblini.net:

On Thu, 8 Feb 2007, David Chelimsky wrote:

--
Bertram Scharpf
Stuttgart, Deutschland/Germany
http://www.bertram-scharpf.de