# Problem on an example from ruby-lang.org

**URL:** <https://www.ruby-forum.com/t/problem-on-an-example-from-ruby-lang-org/195191>\
**Category:** Ruby\
**Created:** [September 18, 2010, 9:17pm UTC](https://www.ruby-forum.com/t/problem-on-an-example-from-ruby-lang-org/195191 "2010-09-18T21:17:27Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![awvalenti](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/779978/32.png) [@awvalenti](https://www.ruby-forum.com/u/awvalenti)\
**Post date:** [September 18, 2010, 9:17pm UTC](https://www.ruby-forum.com/t/problem-on-an-example-from-ruby-lang-org/195191/1 "2010-09-18T21:17:27Z")

</div>

Hello all,

I’m new to Ruby and I’ve been taking a look at the “Getting Started”  
stuff from [ruby-lang.org](http://ruby-lang.org). I found the following example from  
[http://www.ruby-lang.org/en/documentation/ruby-from-other-languages/:](http://www.ruby-lang.org/en/documentation/ruby-from-other-languages/:)

class Fixnum

# You can, but please don’t do this

def +( other )  
self - other  
end  
end

If I run this code using the ruby command, it’s fine. However, on irb, I  
get the following error:

C:/Ruby192/lib/ruby/site\_ruby/1.9.1/rbreadline.rb:8446:in  
`_rl_find_next_mbchar': undefined method `force\_encoding’ for  
nil:NilClass (NoMethodError)  
from C:/Ruby192/lib/ruby/site\_ruby/1.9.1/rbreadline.rb:1692:in  
`block in expand_prompt' from C:/Ruby192/lib/ruby/site_ruby/1.9.1/rbreadline.rb:1676:in `each’  
from C:/Ruby192/lib/ruby/site\_ruby/1.9.1/rbreadline.rb:1676:in  
`expand_prompt' from C:/Ruby192/lib/ruby/site_ruby/1.9.1/rbreadline.rb:1765:in `rl\_expand\_prompt’  
from C:/Ruby192/lib/ruby/site\_ruby/1.9.1/rbreadline.rb:1790:in  
`rl_set_prompt' from C:/Ruby192/lib/ruby/site_ruby/1.9.1/rbreadline.rb:4718:in `readline’  
from C:/Ruby192/lib/ruby/site\_ruby/1.9.1/readline.rb:40:in  
`readline' from C:/Ruby192/lib/ruby/1.9.1/irb/input-method.rb:115:in `gets’  
from C:/Ruby192/lib/ruby/1.9.1/irb.rb:139:in `block (2 levels) in eval_input' from C:/Ruby192/lib/ruby/1.9.1/irb.rb:273:in `signal\_status’  
from C:/Ruby192/lib/ruby/1.9.1/irb.rb:138:in `block in eval_input' from C:/Ruby192/lib/ruby/1.9.1/irb/ruby-lex.rb:188:in `call’  
from C:/Ruby192/lib/ruby/1.9.1/irb/ruby-lex.rb:188:in  
`buf_input' from C:/Ruby192/lib/ruby/1.9.1/irb/ruby-lex.rb:103:in `getc’  
from C:/Ruby192/lib/ruby/1.9.1/irb/slex.rb:205:in `match_io' from C:/Ruby192/lib/ruby/1.9.1/irb/slex.rb:75:in `match’  
from C:/Ruby192/lib/ruby/1.9.1/irb/ruby-lex.rb:286:in `token' from C:/Ruby192/lib/ruby/1.9.1/irb/ruby-lex.rb:262:in `lex’  
from C:/Ruby192/lib/ruby/1.9.1/irb/ruby-lex.rb:233:in `block (2 levels) in each_top_level_statement' from C:/Ruby192/lib/ruby/1.9.1/irb/ruby-lex.rb:229:in `loop’  
from C:/Ruby192/lib/ruby/1.9.1/irb/ruby-lex.rb:229:in `block in each_top_level_statement' from C:/Ruby192/lib/ruby/1.9.1/irb/ruby-lex.rb:228:in `catch’  
from C:/Ruby192/lib/ruby/1.9.1/irb/ruby-lex.rb:228:in  
`each_top_level_statement' from C:/Ruby192/lib/ruby/1.9.1/irb.rb:155:in `eval\_input’  
from C:/Ruby192/lib/ruby/1.9.1/irb.rb:70:in `block in start' from C:/Ruby192/lib/ruby/1.9.1/irb.rb:69:in `catch’  
from C:/Ruby192/lib/ruby/1.9.1/irb.rb:69:in `start' from C:/Ruby192/bin/irb:12:in `’

And then irb abruptly closes. I’m using Windows 7 Professional, ruby  
1.9.2p0 (2010-08-18) [i386-mingw32] and irb 0.9.6(09/06/30).

I’ve done some searching about this problem, but couldn’t find anything.  
I apologize in case it is a known issue.

Regards,  
Andre

---

<div class="post-metadata">

**Author:** ![awvalenti](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/779978/32.png) [@awvalenti](https://www.ruby-forum.com/u/awvalenti)\
**Post date:** [September 18, 2010, 11:17pm UTC](https://www.ruby-forum.com/t/problem-on-an-example-from-ruby-lang-org/195191/2 "2010-09-18T23:17:56Z")

</div>

Andre Willik valenti wrote:

> class Fixnum
> 
> # You can, but please don’t do this
> 
> def +( other )  
> self - other  
> end  
> end

Which bit of “please don’t do this” don’t you understand? 🙂

irb is itself a large and quite complex ruby program; look in  
/usr/lib/ruby/1.8/irb/\* or the equivalent on your system.

If you break basic methods like addition, then it will fail. The same  
would apply to pretty much any other ruby program.

It is, however, a good demonstration of why polluting core classes is a  
really bad idea (because other people’s code will depend on them  
behaving in the usual way, and you can’t isolate those changes so they  
only affect your code)