angularjs - Angular code from ng-book -

i'm reading ng-book , there's code i'm not quite following:

<div ng-controller="mycontroller">   <input ng-model="expr" type="text" placeholder="enter expression" />   <h2>{{ parsedvalue }}</h2> </div>  angular.module("myapp", []) .controller('mycontroller', function($scope, $parse) {   $scope.$watch('expr', function(newval, oldval, scope) {     if (newval !== oldval) {       // let's set our parsefun expression       var parsefun = $parse(newval);       // value of parsed expression       $scope.parsedvalue = parsefun(scope);     }   }); }); 

i'm guessing parsefun() evaluates scope.expr. if case how know evaluate property?

parsefun $parse(newval) in case implying means parsefun new parsed expression. retrieve value of parsed expression passing scope given watch function callback parsefun(scope)

note: in actual angular development find need use watch statement in controller.