How can I get source and variable values in ruby tracebacks?

Question

Here's the last few frames of a typical Ruby on Rails traceback:

And here are the last few frames of a typical Nevow traceback in Python:

It's not just the web environment either, you can make similar comparisons between ipython and irb. How can I get more of these sorts of details in Ruby?

Answer 1

AFAIK, once an exception has been caught it's too late to grab the context in which it was raised. If you trap the exception's new call, you could use evil.rb's Binding.of_caller to grab the calling scope, and do

eval("local_variables.collect { |l| [l, eval(l)] }", Binding.of_caller)

But that's quite a big hack. The right answer is probably to extend Ruby to allow some inspection of the call stack. I'm not sure if some of the new Ruby implementations will allow this, but I do remember a backlash against Binding.of_caller because it will make optimizations much harder.

(To be honest, I don't understand this backlash: as long as the interpreter records enough information about the optimizations performed, Binding.of_caller should be able to work, although perhaps slowly.)

Update

Ok, I figured it out. Longish code follows:

class Foo < Exception
  attr_reader :call_binding

  def initialize
    # Find the calling location
    expected_file, expected_line = caller(1).first.split(':')[0,2]
    expected_line = expected_line.to_i
    return_count = 5  # If we see more than 5 returns, stop tracing

    # Start tracing until we see our caller.
    set_trace_func(proc do |event, file, line, id, binding, kls|
      if file == expected_file && line == expected_line
        # Found it: Save the binding and stop tracing
        @call_binding = binding
        set_trace_func(nil)
      end

      if event == :return
        # Seen too many returns, give up. :-(
        set_trace_func(nil) if (return_count -= 1) <= 0
      end
    end)
  end
end

class Hello
  def a
    x = 10
    y = 20
    raise Foo
  end
end
class World
  def b
    Hello.new.a
  end
end

begin World.new.b
rescue Foo => e
  b = e.call_binding
  puts eval("local_variables.collect {|l| [l, eval(l)]}", b).inspect
end