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Our app is very (JSON) web-service heavy, and so having helpful error messages in our web service documents is pretty important. Luckily, Merb makes this, like everything, a metric shitton easier than it is in rails. There are a couple poorly documented things I had to stumble through, so I thought I would write some up on how to do this.

In Merb, if anything raises an exception, it looks for an action with the same name in the Exceptions controller. merb-gen gave you a simple one in app/controllers/exception.rb. Here's what mine looks like now:

app/controllers/exceptions.rb

class Exceptions < Application
  provides :json                                                  # [1]

  # handle NotFound exceptions (404)
  def not_found
    return standard_error if content_type == :json                # [2]
    render
  end

  # handle NotAcceptable exceptions (406)
  def not_acceptable
    return standard_error if content_type == :json
    render
  end

  # handle NotAuthorized exceptions (403)
  def not_authorized
    return standard_error if content_type == :json
    render
  end

  # Everything else (500)
  def standard_error                                              # [3]
    # Re-Raise so we get the pretty merb error document instead.
    raise request.exceptions.first if content_type == :html       # [4]

    @exceptions = request.exceptions
    @show_details = Merb::Config[:exception_details]
    render :standard_error                                        # [5]
  end

end

Some things to note about what I've done:

1. Make sure it #provides for the web-service content-type.
2. Since I just wanted to use the same view template for every error (see below), I had to explicitly make all web-service calls render that action instead. I could have just removed those methods and deleted the templates, but then any html views in the app would be generic, and I wanted custom ones for 403 and 404.
3. Since all errors inherit from StandardError, this will catch everything.
4. However, in the case of HTML documents, we want to use the fancy merb one, so re-raise the error so that merb's default error controller will handle it for us.
5. Set up some variables to use in the view, then render that. Be sure to include :standard_error so that the other error handlers know which template to render!

And finally, here's what my view looks like. You can do whatever you want, of course. #j is just a global helper I have that basically does #to_json on whatever you give it, with some special cases for date formatting and indenting in dev vs production.

app/views/exceptions/standard_error.json.erb:

<%= j( {
  :_type        => "InternalServerError",
  :request_uri  => request.env['REQUEST_URI'],
  :parameters   => params,
  :exceptions   => @exceptions.map do |exception|
    {
      :name       => exception.class,
      :message    => exception.message,
      :backtrace  => @show_details ? exception.backtrace : exception.backtrace.first.to_a
    }
  end
} ) %>