From: Brian Candler <B.Candler@...>
Date: 2003-04-24T16:54:58+09:00
Subject: Re: two oddities in ruby 1.8.0p2

On Thu, Apr 24, 2003 at 08:04:28AM +0900, Dave wrote:
> Another oddity is one that I think has been around for a while, but I
> haven't seen it discussed before (correct me if I'm wrong -- I'm new
> to ruby).

Oh, it has been discussed many a time :-)

> It's the infamous local variable scope issue. In the
> discussions I've read, methods are supposed to define a scope for
> local variables. Blocks within methods also have their own scope, if
> the variables have not been used already. BUT control structures like
> if-else, for, while, etc. are NOT supposed to define their own scope.
> How, then, to explain this example?
> 
> i = 0
> while i < 10
> 	puts x if defined? x
> 	x = 10
> 	i += 1
> end
> 
> Nothing is outputted.

It's nothing to do with blocks. It's because of the method/local variable
ambiguity.

The decision "is x a local variable or is it a method?" is made
*statically*, at compile time, as the code is being read in and turned into
a syntax tree. The rule is that if any statement of the form 'x = ...' has
been seen earlier, then a standalone 'x' is treated as a local variable, not
a method call.

In your example, at the line 'puts x if defined? x', no local variable
assignment to x has been seen previously. Therefore this is compiled as a
method call:

    puts self.x if defined? self.x

which always fails since you never define a method 'x'

You can see it in this example:

   def x
     33
   end
   i = 0
   while i < 10
     puts x        # compiled as puts self.x
     x = 99
     puts x        # compiled as puts x (the local variable)
     i += 1
   end

which prints 33 99 33 99 33 99 ...

The assignment does not even have to be executed:

   def y
     33
   end
   y = 99 if false
   p y             # prints 'nil'

The last reference to 'y' is interpreted as a local variable because an
assignment to y was seen earlier during the parsing of this method, even
though at run-time it never gets executed.

Some other discussions are archived here:
http://www.rubygarden.org/ruby?LocalVariablesAndBlocks

Cheers,

Brian.