angularjs - Non-assignable error in directive when using ternary operator -

i have directive has isolate scope action property. when using ternary operator in view, i'm getting expression 'ctrl.someboolean ? 'some text' : 'different text'' used directive 'mydirective' non-assignable!

usages

 <my-directive name='my name' action="ctrl.someboolean ? 'some text' : 'different text'">   <my-directive name='my name 2' action="'some static text'"> 

directive

 angular.module('example')   .directive('mydirective', [     function() {       return {         restrict: 'e',         replace: true,         transclude: true,         templateurl: 'mydirective.html',         scope: {           name: '@',           action: '=?'         },         require: ['mydirective', 'form'],         controller: 'mydirectivectrl',         controlleras: 'ctrl',         bindtocontroller: true       };     }]); 

directive controller

angular.module('example')   .controller('mydirectivectrl', ['$scope', '$element', '$attrs',     function($scope, $element, $attrs) {       var vm = this;        vm.action = angular.isdefined($attrs.action) ? $attrs.action : 'default action';     }   ]) 

from way using directive, better not use two-way binding. need simple one-way attribute interpolation, i.e. directive usage should be:

<my-directive name='my name' action="{{ ctrl.someboolean ? 'some text' : 'different text' }}"></my-directive> 

and directive scope configuration

scope: {     name: '@',     action: '@?' }, 

why angular throws error if use two-way binding ternary expression? easy understand if think two-way binding means. primary purpose of allow directive scope modify original outer scope value. however, not possible assign new value expression ctrl.someboolean ? 'some text' : 'different text'. hence error.