From: Michael Winterstein <parzival@...>
Date: 2009-11-29T03:15:24+09:00
Subject: Re: Local variables can't be accessed from outside, right?

Benoit Daloze wrote:

> 
> I didn't find other way to get the result of class A ... end like a if 
> or
> case result.
> Very weared a class' instance variable(1)
> Does it have any interest?
> 

No, it's just an example for learning about metaprogramming.  Which is 
why it seems strange; the local variables can only be 'accessed' because 
they happen to be return values.

I did learn something I didn't know, however :

class A
    puts "Local variables: #{local_variables}"
    puts "Instance variables: #{instance_variables}"
    a = 5
    @a = 6
    puts "Local variables: #{local_variables}"
    puts "Instance variables: #{instance_variables}"
end

Local variables: a
Instance variables:
Local variables: a
Instance variables: @a

So local variables exist in some ethereal void after the code is scanned 
but before they can be used; not so with instance variables.  That makes 
some sense.  But this also lets you do this eval weirdness.

class B
    puts "#{local_variables.first} = #{eval(local_variables.first)}, 
which is a  #{eval(local_variables.first).class}"
    eval(local_variables.first + "=2")
    puts "#{local_variables.first} = #{eval(local_variables.first)}, 
which is a  #{eval(local_variables.first).class}"

    puts #{a} if a    #=> NameError: undefined local variable or method 
'a' for B:Class
    a = 4
    puts "#{local_variables.first} = #{eval(local_variables.first)}, 
which is a  #{eval(local_variables.first).class}"
end

With the error commented out, this gives:
a = , which is a  NilClass
a = 2, which is a  Fixnum
a = 4, which is a  Fixnum


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