MyApp = lambda do |env|
  if resp = env["rack.openid.response"]
    case resp.status
    when :success
      ...
    when :failure
      ...
  else
    [401, {"WWW-Authenticate" => 'OpenID identifier="http://example.com/"'}, []]
  end
end

use Rack::OpenID
run MyApp

# Session needs to be before Rack::OpenID
use Rack::Session::Cookie

require 'rack/openid'
use Rack::OpenID

get '/login' do
  erb :login
end

post '/login' do
  if resp = request.env["rack.openid.response"]
    if resp.status == :success
      "Welcome: #{resp.display_identifier}"
    else
      "Error: #{resp.status}"
    end
  else
    headers 'WWW-Authenticate' => Rack::OpenID.build_header(
      :identifier => params["openid_identifier"]
    )
    throw :halt, [401, 'got openid?']
  end
end

enable :inline_templates

__END__

@@ login
<form action="/login" method="post">
  <p>
    <label for="openid_identifier">OpenID:</label>
    <input id="openid_identifier" name="openid_identifier" type="text" />
  </p>

  <p>
    <input name="commit" type="submit" value="Sign in" />
  </p>
</form>