From: "Rishel,Wes" <Wes.Rishel@...>
Date: 2007-07-02T04:18:45+09:00
Subject: Re: Newbie Question. eval.rb variable scope issue

Stefano, mil grazie!

I don't know how to pass a suggestion to those who maintain the library, but it would be a one-line change to make the "simple" one use while 1 instead of loop do.





-----Original Message-----
From: Stefano Crocco [mailto:stefano.crocco@alice.it] 
Sent: Sunday, July 01, 2007 12:10 PM
To: ruby-talk ML
Subject: Re: Newbie Question. eval.rb variable scope issue

Alle domenica 1 luglio 2007, Rishel,Wes ha scritto:
> It is reassuring to know that the problem is a local problem.

It's not a local problem. As I said in my second message, there are two files 
called eval.rb involved in this problem. One is included in the ruby 
distribution, is the one you're using and doesn't allow to run the code in 
the tutorial; the other is the one in the tutorial website 
(http://www.ruby-doc.org/docs/UsersGuide/rg/eval.txt), which is the one I 
used, which works correctly. The latter, as the tutorial itself says, is more 
advanced, but the main difference is that it uses 'while', while the other 
uses 'loop'. Since 'loop' introduces a new scope, variables defined in one 
iteration are lost in the next - so the bug. 'while' doesn't introduce a new 
scope, so the program works.

> Generally, the scope of a local variable is one of
>
>         proc{ ... }
>         *** loop{ ... } *** (my emphasis)
>         def ... end
>         class ... end
>         module ... end
>         the entire program (unless one of the above applies)

You're mostly right here. proc and loop are instance methods of the Kernel 
module, so they don't create a new scope. What happens is that you pass these 
methods a block, and it is the block which introduces a new scope (the same 
happens for every method which accepts a block).

I hope this helps

Stefano

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