This post originated from an RSS feed registered with Java Buzz by Brian McCallister. Original Post: Ruby Metaprogramming 201 Feed Title: Waste of Time Feed URL: http://kasparov.skife.org/blog/index.rss Feed Description: A simple waste of time and weblog experiment Latest Java Buzz Posts Latest Java Buzz Posts by Brian McCallister Latest Posts From Waste of Time

If you find yourself doing a lot of eval(..) based metaprogramming, remember to include debug stuff!

begin
    eval("dsadh!!sadkhj how are you today?")
rescue Exception => e
    log.error "Metaprogramming error [#{e}]"
end

Which is nifty and all if the eval fails, but what happens on the way more subtle?

begin
  eval <<-EOM
    def foo
      robert is sick today
    end
  EOM
rescue Exception => e
  log.error "Metaprogramming error [#{e}]"
end

Where you don't get the syntax problem until you invoke foo. Solution!

eval <<-EOM
  def foo
    begin
      robert is sick today
    rescue Exception => e
      puts "Error while invoking foo defined from [insert stuff to help you debug]: \#{e}"
    end
  end
EOM 

You cannot catch syntax errors, but because so much of ruby uses ruby this isn't a big deal most of the time =) Note escaping the hash in \#{e} -- this is important because just #{e} would fail. Why is an exercise left to the reader ;-)