From: "Iñaki Baz Castillo" <ibc@...>
Date: 2011-04-23T03:15:25+09:00
Subject: Is it usual/valid to extend custom classes under Errno module?

Hi, I've coded a DNS library. When a DNS query fails my library
doesn't raise an exception (as it's expensive) but instead returns a
Symbol (more efficient):

- :dns_error_nxdomain - The domain name does not exist.
- :dns_error_nodata - There is no data of requested type found.
- :dns_error_tempfail - Temporary error, the resolver nameserver was
not able to process our query or timed out.
- :dns_error_protocol - Protocol error, a nameserver returned malformed reply.


An example usage:

-----------------------------------------------------------
resolver = EM::Udns::Resolver.new
EM::Udns.run resolver

query = resolver.submit_A "google.com"

query.callback do |result|
  puts "result => #{result.inspect}"
end

query.errback do |error|
  case error
  when :dns_error_nxdomain
    # do something
  when :dns_error_nodata
    # do something
  when :dns_error_tempfail
    # do something
  end
end
--------------------------------------------------------------

So returning a Symbol (in case of failure) is good as I can use it
within a case/when statement.

Another possibility would be returning an instance of a class, something like:

  class EM::Udns::ErrorNoDomain <  EM::Udns::Error ; end
  class EM::Udns::ErrorNoData     <  EM::Udns::Error ; end
  class EM::Udns::ErrorTempFail  <  EM::Udns::Error ; end


or I could extend Errno module:

  class Errno::DnsNoDomain ; end
  class Errno::DnsNoData     ; end
  class Errno::DnsTempail    ; end


In your opinnion, which is the most elegant way? any other suggestion?

Thanks a lot.

-- 
Iñaki Baz Castillo
<ibc@aliax.net>