Forum: Ruby Object#instance_eval but better

Announcement (2017-05-07): www.ruby-forum.com is now read-only since I unfortunately do not have the time to support and maintain the forum any more. Please see rubyonrails.org/community and ruby-lang.org/en/community for other Rails- und Ruby-related community platforms.
Richard C. (Guest)
on 2008-10-24 21:56
(Received via mailing list)
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?

Is this possible without reading the file into a string?  Is this
possible without hacking the C code?  Should I go bother ruby-core?

Thanks,

Richard
Thomas S. (Guest)
on 2008-10-24 22:09
(Received via mailing list)
On Oct 24, 1:55 pm, Richard C. <removed_email_address@domain.invalid> wrote:
> file executed as if it were the body of a method in a class.  The file
>      def initialize(path)
> 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.
Ken B. (Guest)
on 2008-10-26 04:10
(Received via mailing list)
On Fri, 24 Oct 2008 13:08:45 -0500, Trans wrote:

>> string and then eval'ing that string just to get the contents of the
>>
>> 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.

>       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
Ara H. (Guest)
on 2008-10-26 06:42
(Received via mailing list)
On Oct 24, 2008, at 11:55 AM, Richard C. wrote:

> 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:


reverse your logic: don't force the code to eval inside a context -
provide the context.




cfp:~ > cat a.rb
class Foo
   class << self
     attr_accessor :current

     def initializing &block
       caller = eval 'self', block

       if caller.is_a?(Foo)
         previous = current
         begin
           Foo.current = caller
           block.call
         ensure
           Foo.current = previous
         end
       else
         current.instance_eval &block
       end
     end
   end

   def initialize path
     Foo.initializing{ load path }
   end
end

foo = Foo.new('b.rb')
p foo.instance_variables




cfp:~ > cat b.rb
Foo.initializing do
   @foo = 42
end



cfp:~ > ruby a.rb
["@foo"]





a @ http://codeforpeople.com/
This topic is locked and can not be replied to.