exception_mailer.rb

require 'sinatra/base'
module Sinatra
  # module to catch Sinatra errors and send a email
  module ExceptionMailer
    def initialize(app)
      @app = app
      # set parameters here..
      yield self if block_given?
    end

    def call(env)
      status, headers, body =
        begin
        @app.call(env)
      rescue => error_raised
        # TODO don't allow exceptions from send_notification to
        # propogate
        send_notification error_raised, env
       raise
      end
      [status, headers, body]
    end

    private

    def send_notification(exception, env)
      puts "SENDING NOTIFICATION for :#{exception}"
    end
  end
  
  register ExceptionMailer
end

output.txt

Request "/" should get 'hello mate..' but instead, the server shows an error, and doesn't respond to request.

== Sinatra/0.9.5 has taken the stage on 4567 for development with backup from Thin
>> Thin web server (v1.2.7 codename No Hup)
>> Maximum connections set to 1024
>> Listening on 0.0.0.0:4567, CTRL+C to stop
SENDING NOTIFICATION for :private method `call' called for nil:NilClass
!! Unexpected error while processing request: undefined method `each' for nil:NilClass

test_exception_mailer.rb

# simple script to test catching a sinatra exception
require 'rubygems'
require 'sinatra'
require 'exception_mailer'

configure do
  set :raise_errors, true
  #  enable :raise_errors
  #  set :show_exceptions, false
end

get '/' do
  "hello mate!  Try <a href='/boom'>this link</a>"
end

get '/boom' do
  raise 'uh oh.. mate! we got a problem. '
end

error do
  'Sorry there was a nasty error - ' + env['sinatra.error'].name
end