From: Gavin Kistner <gavin@...>
Date: 2005-08-19T14:50:38+09:00
Subject: Redefining Kernel Methods on the Fly

As part of my Quest to find a clean way to get #puts and #print to  
'work' in ERB, I was experimenting with ways I thought I could  
redefine Kernel methods from within a method. Why don't test1 and  
test2 work in the following code?

module Kernel
         alias_method :_puts, :puts
         def puts(s); _puts "HIDDEN"; end
end
puts "1: I DON'T WANNA SEE THIS"
module Kernel
         undef_method :puts
         alias_method :puts, :_puts
end
puts "1: OK to see this"

def run_test1
         Kernel.instance_eval{
                 alias_method :_puts, :puts
                 def puts(s); _puts "HIDDEN"; end
         }
         puts "2: I DON'T WANNA SEE THIS"
         Kernel.instance_eval{
                 undef_method :puts
                 alias_method :puts, :_puts
         }
         puts "2: OK to see this"
end
run_test1

def run_test2
         kb = Kernel.instance_eval{ binding }
         eval( "alias_method :_puts, :puts", kb )
         eval( 'def puts(s); _puts "HIDDEN"; end', kb )
         puts "3: I DON'T WANNA SEE THIS"
         eval( "undef_method :puts", kb )
         eval( "alias_method :puts, :_puts", kb )
         puts "3: OK to see this"
end
run_test2

[[Slim:~/Desktop] gavinkis% ruby -vW0 binding_test.rb
ruby 1.8.2 (2004-12-25) [powerpc-darwin8.0.0]
HIDDEN
1: OK to see this
2: I DON'T WANNA SEE THIS
2: OK to see this
HIDDEN
HIDDEN