From: Brian Candler <b.candler@...>
Date: 2010-02-20T20:28:57+09:00
Subject: Re: Ruby conditionals subtlety?

Maybe it's clearer like this:

  if false
    foo = 123
  end
  puts foo      # nil

  puts bar      # undefined local variable or method 'bar'

That is, for a bare word expression like 'foo' ruby has to decide 
whether to parse it as a method call - as foo() or self.foo - or as a 
local variable reference.

It makes the decision based on whether there has been a previous 
assignment of the form "foo = ..." parsed earlier in the code. This is 
regardless of whether the code is actually executed, because we haven't 
started executing any of it yet.

"earlier" in the code is strictly left-to-right.

   foo = 123 if not defined?(foo)

At the point of defined?, "foo = ..." has already been seen, and so 
bareword foo is known to be a local variable, and therefore the symbol 
is 'defined' as a local variable at this point in the source code, 
whether or not an assignment has actually been made.
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