From: ts <decoux@...>
Date: 2002-08-15T22:05:14+09:00
Subject: Re: adding method to existing class

>>>>> "E" == Eugene Scripnik <Eugene.Scripnik@itgrp.net> writes:

E>   Now blah responds to some_method. Is there way to include methods
E>   without eval?

 You must understand that there is nothing wrong in #eval, this is just
 that some times #eval is evil :-)

 You have, mainly, 2 form for #eval

  * eval "some_string"

    if you *trust completely* (for example you have hardcoded it in the
    script) the string there is no problem.

    It can exist a problem when you try to use #eval with any strings (even
    strings that you don't know the content) and this is where reside the
    evil side of #eval, because eval is very powerfull and can do anything.

    In this case you can always try to protect you with $SAFE, something
    like this

    Thread.new do
       $SAFE = 4
       eval string
    end.join

  * eval { ... }

    If you use this form, you have fatally hardcoded the block and this mean
    that you can, perhaps, trust it.

 #eval is powerfull, but it's sometimes too powerfull (its evil side) or
  overkill and in many cases you can just use another form, like __send__

 For you example you can write

pigeon% cat b.rb
#!/usr/bin/ruby
some_class = Struct.new( 'SomeClass', 'some_field' )
blah = some_class.new( 'anything' )

module Feature
   def some_method
      puts "some_method"
   end
end

Feature.__send__(:append_features, some_class)
blah.some_method
pigeon% 

pigeon% b.rb
some_method
pigeon% 


 You just want to use include, no need for the evil eval just send the
 method #append_features to the module

 Use the evil eval only when it don't exist another way to do it and when
 it's secure to use this evil eval


Guy Decoux