Using functions from directive controller within link function of same directive

Perhaps I have a fundamental misunderstanding of how directive controllers work, from what I understand they are used as a sort of API to be exposed to other directives & controllers. I am trying to get the controller and link function to communicate internally.

For example I would like to be able to set a variable via the controller function and then use it in the link function:

var app = angular.module('test-app', []);

app.directive('coolDirective', function () {
    return {
        controller: function () {
            this.sayHi = function($scope, $element, $attrs) {
                $scope.myVar = "yo"
            }
        },
        link: function(scope, el, attrs) {
            console.log(scope.myVar);
        }   
    }
});

How can I access myVar or sayHi within the link function? Or have I just missed the point completely?