Re: Object#instance_eval but better

On Fri, 24 Oct 2008 13:08:45 -0500, Trans wrote:

> On Oct 24, 1:55 pm, Richard Crowley <[e-mail]> wrote:
>> I have a bit of code here that I'd love to make better:
>>
>> class Foo
>>      def initialize(path)
>>          self.instance_eval(File.read(path))
>>      end
>> end
>>
>> I don't want to continue down the path of reading an entire file into a
>> string and then eval'ing that string just to get the contents of the
>> file executed as if it were the body of a method in a class.  The file
>> passed to the constructor above might contain something like:
>>
>> @foo = 'This will become an instance variable when you do
>> Foo.new(path)'
>>
>> I've tried all manner of hacks to make Kernel#load and friends execute
>> code as if it were in the calling scope.  In every case I've seen the
>> loaded file leave no lasting effect on the instance variables of the
>> calling method.  My goal is to be able to do exactly this:
>>
>> class Foo
>>      def initialize(path)
>>          load_in_scope path
>>      end
>> end
>> f = Foo.new('load_me.rb')
>> f.instance_variables # => ['@foo']
>>
>> My assumption here is that reading a file into a string and eval'ing it
>> is very slow and is bad form.  Am I right?

I can't imagine that it would be slower than simply loading the ruby 
file. If there's a hint of bad form in it (and I'm not personally going 
to say whether there is or isn't), it would have to relate to security 
implications. And the only way to comment on that is to ask what exactly 
you're trying to do with this construction.

>> Is this possible without reading the file into a string?  Is this
>> possible without hacking the C code?  Should I go bother ruby-core?
> 
> Ultimately, it's the same thing, whether you do it in Ruby or C. I doubt
> it would be that much faster in C than it is in pure Ruby b/c it is
> dynamic --the path can change. However, you may actually intend:
> 
>   class Foo
>       def initialize(path)
>           eval(File.read(path), __FILE__, __LINE__)
>       end
>   end
> 
> T.

He almost certainly doesn't intend __FILE__, __LINE__ because that is the
default behavior of eval. (and besides it won't work since the second 
parameter has to be a binding)

irb(main):014:0> eval "puts __FILE__\nfail", __FILE__
TypeError: wrong argument type String (expected Proc/Binding)
	from (irb):14:in `eval'
	from (irb):14
	from :0


Propably he wants
eval(File.read(path),binding,path)

(the filename and line number passed indicate the location used for 
__FILE__, __LINE__ and exception stack traces that occur in the eval'ed 
code

irb(main):013:0> eval "puts __FILE__\nfail", binding, 'foo'
foo
RuntimeError: 
	from foo:2
	from (irb):13
	from :0

-- 
Chanoch (Ken) Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu/~kbloom1/