angularjs - ng-show directive not working properly -

i using ng-show directive show/hide alert box in ui:

<alert ng-show="{{showalert()}}">{{showalert()}}</alert>

my controller code is:

$scope.showalert = function () { return userform.$invalid || false; };

though getting correct value content of alert , able see booelan values in ui alert still keeps on showing.

any idea?

1. ng-show accepts expression.
2. userform lives in $scope.

template:

<span ng-show="showalert()">form invalid!!</span> 

controller:

$scope.showalert = function() {   return $scope.userform.$invalid || false; }; 

or, simply:

<span ng-show="userform.$invalid">form invalid!!!</span> 

and link working example: http://plnkr.co/edit/nlc1dbtk8vazbbgjnlb0