better_em_defer?.rb

require 'rubygems'
require 'eventmachine'

module EventMachine
  # This returns a deferrable and catches errors in the block for you.
  #
  # Define callbacks and errbacks in usual way instead of weird EM.defer way
  def self.better_defer?(&block)
    deferrable = EM::DefaultDeferrable.new

    EM.defer do
      begin
        output = block.call
        deferrable.succeed output
      rescue => e
        deferrable.fail e
      end
    end

    deferrable
  end
end

# Example

EM.run {
  deferrable1 = EM.better_defer? do
    sleep 1
    'done'
  end

  deferrable2 = EM.better_defer? do
    raise "woops"
  end

  deferrable1.callback do |output|
    puts "Deferrable 1 succeeded with #{output}"
  end

  deferrable2.errback do |ex|
    puts "Deferrable 2 failed: #{ex.class} #{ex.message}"
  end
}