From: Stefano Crocco <stefano.crocco@...>
Date: 2008-07-23T19:18:12+09:00
Subject: Re: circular 'require'

On Wednesday 23 July 2008, Shadowfirebird wrote:
> Hi,
>
> I'm new to ruby and I'm really enjoying it.   However, there is one
> thing...
>
> I gather from this list and my poor efforts that ruby does not like
> files that 'require' each other circularly.  I can find nothing in the
> documentation about this, though; in fact, the description of require
> seems to actually rule it out.   Can anyone explain what is going on
> here?
>
> (This happens in 1.8 and 1.9 and JRuby 1.0.  So it's definitely me,
> not Ruby, that has the problem.)
>
>
> To flesh things out a little:
>
> 'require' is supposed to load files, unless they have been already
> loaded.  It stores an array of loaded files in $" for this purpose.
>
> Okay, suppose we have two programs in two different files:
>
> # test1.rb
> require "test2.rb"
> class One
>   def self.testone(); Two.whoistwo; end
>   def self.whoisone();  puts "class one"; end
> end
>
> # test2.rb
> require "test1.rb"
> class Two
>   def self.testtwo(); One.whoisone; end
>   def self.whoistwo(); puts "class two"; end
> end
>
> >ruby -w test1.rb
>
> ./test1.rb:4: unititialized constant Two (NameError)
>
> Why does this happen?  I would have thought that test1 would load
> test2; then test2 would go to load test1 but find it already loaded.
> Apparently what happens is that test2 doesn't get loaded at all.

The required file is added to $" only after its contents have been processed, 
so when the line

require 'test1.rb'

is executed, the file 'test1.rb' hasn't as yet inserted into $". To be more 
specific, here's what happens when the line

require 'test1.rb'

is executed:

* the file test1.rb is read and parsed
* starts the execution of test1.rb
* the line require 'test2.rb' is executed. This means:
  * the file test2.rb is read and parsed
  * starts the execution of test2.rb
  * the line require 'test1.rb' is executed. This means:
    * the file test1.rb is read and parsed
    * starts the execution of test1.rb
    * the line require 'test2.rb' is executed. This means:
      ...

As you can see, this leads to an endless loop.

Can I ask you why do you need these circular requires? It's a situation which 
seldom happens using ruby.

Also, using require, you don' need to specify the .rb extension.

I hope this helps

Stefano