AngularJS – Injecting factory from another module into a provider

I think is because all the providers are instantiated before the factories and so a provider has to depend only on other providers.

As a way around that, I am using the injector method of angular.module to create the module.
A plunker that should do what you were trying to accomplish: http://plnkr.co/edit/g1M7BIKJkjSx55gAnuD2

Notice that I changed also the factory method. The factory method is now returning an object
with a connect method.

var angularSocketIO = angular.module('socketioModule', ['ng']);
angularSocketIO.factory('socketio', [
    '$rootScope',
    function($rootScope) {
      return {
        connect: function(addr) {
          var socket = io.connect(addr, {
            'sync disconnect on unload': true
          });

          return socket;
        }
      };
    }]);


  angular.module('myApp.services', ['socketioModule'])
  .provider('greeter', [
    function() {
      var injector = angular.injector(['socketioModule']);
      var socketio = injector.get('socketio');

      var salutation = 'Hello';
      this.setSalutation = function(s) {
        salutation = s;
      }

      function Greeter(a) {
        this.salutation = salutation;
        socket._emit('hello');

        this.greet = function() {
          return salutation + ' ' + a;
        };
      }

      this.$get = function(version) {
        return new Greeter(version);
      };
    }
  ]);


  var myApp = angular.module('myApp', ["myApp.services"]);