From: Mauricio Fernandez <mfp@...>
Date: 2006-07-30T20:05:57+09:00
Subject: Re: eval bind

On Sun, Jul 30, 2006 at 07:44:06PM +0900, Karel Michek wrote:
> This code:
> 
> puts eval("var" ).class
> puts eval("var").class.instance_methods(false).sort
> var = "";
> 
> produces:
> 
> NilClass
[...]
> 
> and this code
> 
> puts eval("var" ).class
> puts eval("var").class.instance_methods(false).sort
> 
> produces
> 
> undefined local variable or method `var' for main:Object (NameError)
> 
> which is exactly what I would expect in the first case also.

When the "var" String is eval()ed, var="" has already been parsed. Ruby
already knows that var refers to a local variable, so "var" will be
interpreted correspondingly. When this happens, var hasn't been assigned to,
though, so the returned value is nil.

This is what's happening:

eval("a")                                          # => nil
a = 1   # since the parser has seen this, "a" will be a local
        # in code parsed from now on. Its value is nil until it is
	# initialized.

Compare it to the following situation, where the block is parsed before you
assign to a:

instance_eval { a }                                # => 
a = nil
# ~> -:1: undefined local variable or method `a' for main:Object (NameError)
# ~> 	from -:1

-- 
Mauricio Fernandez  -   http://eigenclass.org   -  singular Ruby