From: Brian Candler <b.candler@...>
Date: 2009-08-11T05:33:46+09:00
Subject: Re: instance_variables vs. local_variables

David Whetstone wrote:
> So the implication here is that there is no way to define a local 
> variable at run time.

There's eval with a binding. But the question is, why would you want to 
define a local variable at run time? Whatever problem you're trying to 
solve, it sounds like you're trying to solve it the wrong way.

> I can use method_missing to simulate late binding of local variables, 
> but it is inefficient to have method_missing called every time such a 
> variable is referenced.

I think you've just explained why Ruby works this way.

A bare "foo" could be a method call, self.foo, or a local variable 
reference. If this were done dynamically, every single use of "foo" 
would be expensive. Even "foo = foo + 1" might be a self.foo on the RHS 
the first time it was called.

But the way Ruby choses to resolve this, it knows up-front whether foo 
is just a cheap offset into the stack frame.

  if false
    foo = nil
  end
  foo      #  foo *is* a local variable
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