From: "Ara.T.Howard" <Ara.T.Howard@...>
Date: 2005-07-20T01:40:23+09:00
Subject: Re: Instance Eval of a Proc

On Wed, 20 Jul 2005, Jacob Fugal wrote:

> On 7/19/05, Ara.T.Howard <Ara.T.Howard@noaa.gov> wrote:
>> On Wed, 20 Jul 2005, Gavin Kistner wrote:
>>
>>> [Slim:~/Desktop] gavinkis% cat instance_eval_proc.rb
>>>   class Foo
>>>     def self.show_name( *args )
>>>       puts "Foo.show_name says that @name is '#@name' for #{self.inspect}"
>>>     end
>>>     def initialize( name )
>>>       @name = name
>>>     end
>>>   end
>>>
>>>   f = Foo.new( "Jim" )
>>>
>>>   my_lambda = lambda{
>>>     puts "my_lambda says that @name is '#@name' for #{self.inspect}"
>>>   }
>>>   p my_lambda
>>>   f.instance_eval( &my_lambda )
>>>
>>>   my_proc = Proc.new{
>>>     puts "my_proc says that @name is '#@name' for #{self.inspect}"
>>>   }
>>>   p my_proc
>>>   f.instance_eval( &my_proc )
>>>
>>>   meth_lambda = Foo.method( :show_name ).to_proc
>>>   p meth_lambda
>>>   f.instance_eval( &meth_lambda )
>>
>> just because you are call meth_lambda in instance_eval doesn't mean
>> @name will be defined.
>
> I think Gavin's point is that it *does* work for my_lambda and
> my_proc. meth_lambda is the only exception. Neither my_lambda nor
> my_proc knew about @name either when they were defined. instance_eval
> bound self to the caller for the evaluation of the proc. However, the
> same binding didn't take place for meth_lambda, and it used Foo as
> self instead of f.

well:

   harp:~ > ri Object.method

   ---------------------------------------------------------- Object#method
        obj.method(sym)    => method
   ------------------------------------------------------------------------
        Looks up the named method as a receiver in _obj_, returning a
        +Method+ object (or raising +NameError+). The +Method+ object acts
        as a closure in _obj_'s object instance, so instance variables and
        the value of +self+ remain available.
   ...
   ...
   ...

so i don't really see the exception - the behaviour of Foo.method(:show_name)
does exactly as advertised - binding the method to the class Foo along with
that notion of self and instance vars.  so the binding of self for the
duration of instance_eval is definitely working - what gavin is attempting to
do is akin to hoping

   f.instance_eval do
     p ::File::LOCK_EX
   end

would somehow print some other class/objects constant named 'LOCK_EX' in that
the caller/scope-resolution for show_name is already set after the call to
Foo::method.  in the cases like

   f.instance_eval &my_proc

it has not already been set and is then set for the call.  eg. Foo::method
binds exactly as f.instance_eval does - only a layer deeper.

anyhow it seems consistent - but perhaps not intuitive.

kind regards.


-a
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