class IRB::ExtendCommand::ShowSource

def execute(str = nil)
  unless str.is_a?(String)
    puts "Error: Expected a string but got #{str.inspect}"
    return
  end
  source = self.class.find_source(str, @irb_context)
  if source && File.exist?(source.file)
    show_source(source)
  else
    puts "Error: Couldn't locate a definition for #{str}"
  end
  nil
end