From: Kent Dahl <kentda+news@...>
Date: 2003-12-13T03:47:01+09:00
Subject: Re: Simple issue giving problems

gabriele renzi wrote:
> on a sidenote, I've been playing with:
> 
> def swapper(obj,a,b)
>  obj.instance_eval("#{a},#{b}=#{b},#{a}")
> end
> 
> swapper(self,:a,:b)
> 
> 
> but this does'nt work.

Because the scope of the variables is different, I believe. The method 
does not have access to local variables of the calling method. Even if 
you use instance_eval, you will just have access to instance variables 
of the given self, not local variables for any arbitrary local method 
called on it further up the callchain.

> Anyway, if I do this:
> 
> p a
> p b
> 
>  self.instance_eval("#{:a},#{:b}=#{:b},#{:a}")
> p a
> p b
> 
> it works as expected (i.e. swaps the variables)
> 
> Some hints to explain me this ?


a = 3
b = 5

# Will not work, different scopes.
def swapper(obj,a,b)
  obj.instance_eval("#{a},#{b}=#{b},#{a}")
end
swapper(self,:a,:b)
print a, b, "\n"         #=> 35

# Pass scope deliberately as a Proc.
def swapper2(a,b,&block)
   eval("#{a},#{b}=#{b},#{a}", block)
end
swapper2(:a,:b){}
print a, b, "\n"         #=> 53


You can do a lot of fancy (but ugly) things with the latter form. I tend 
to rewrite it to pass arguments through the Proc, just to make it look 
as ugly as it really is.

# Pass scope deliberately as a Proc.
def swapper3(&block)
   a,b = block.call
   eval("#{a},#{b}=#{b},#{a}", block)
end
swapper3{[:a,:b]}
print a, b, "\n"         #=> 35


HTH

-- 
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