angular directive data binding

example

/*

  I have a directive (errorhandler, see below) that is restricted to Element and Attribute
  Currently, I've got two separate two way bindings, and two separate watchers: 
  one for the element case, one for the attribute case

  ** Is there a way to conditionally set the directives isolate scope?
  
  scope : {
    errors : "=????" //use errorhandler value if available, else use @error value
  }
  Instead of:
  scope : {
    case1 : "=errorhandler" #the Element case
  , case2 : "=errors" #the Attribute case
  }
 
  I'd like to do something like:
  scope : {
    errors : "=errorhandler || errors" #look for errorhandler else 'fall back' to looking for attr.errors
  }
*/

#case1
input(type="text", ng-model="some.thing", errorhandler="some.$errors.thing")

#case2
div.someothercontrol
errorhandler(errors="some.$errors.otherthing")

# directive
angular.module('myapp', [])
  .directive('errorhanlder', function () {

  use strict';
  return {
    restrict: "EA"
    , replace: true
    , transclude: true
    , scope : {
        tag  : "=ofpErrors"
      , prop : "=errors"
    }
    , link : function(scope, el, attrs){

        var errbox = $compile('<ul class="ofp-errors" ng-show="errors"></ul>')(scope)
          , handler = function(newerrors, olderrors){
              scope.errors = newerrors;
              if(!newerrors) return;
              var error_lis = '<li ng-repeat="err in errors">{{ err }}</li>';
              $compile(errbox.html(error_lis))(scope);
          };

          //handle the case where the directive is invoked...
          if(el[0].tagName.toLowerCase() === 'ofp-errors'){
            //... from a tag
            el.replaceWith(errbox)
            scope.$watchCollection('prop', handler);
          } else {
            //... from an attribute
            el.after(errbox);
            scope.$watchCollection('tag', handler);
          }
    }
  }
}