How do I add information to an exception message without changing its class in ruby?

Question

How do I add information to an exception message without changing its class in ruby?

The approach I'm currently using is

strings.each_with_index do |string, i|
  begin
    do_risky_operation(string)
  rescue
    raise $!.class, "Problem with string number #{i}: #{$!}"
  end
end

Is there a better way?

Answer 1

It's not much better, but you can just reraise the exception with a new message:

raise $!, "Problem with string number #{i}: #{$!}"

You can also get a modified exception object yourself with the exception method:

new_exception = $!.exception "Problem with string number #{i}: #{$!}"
raise new_exception

Answer 2

My approach would be to extend the rescued error with an anonymous module that extends the error's message method:

def make_extended_message(msg)
    Module.new do
      @@msg = msg
      def message
        super + @@msg
      end
    end
end

begin
  begin
      raise "this is a test"
  rescue
      raise($!.extend(make_extended_message(" that has been extended")))
  end
rescue
    puts $! # just says "this is a test"
    puts $!.message # says extended message
end

That way, you don't clobber any other information in the exception (i.e. its backtrace).