foo.bar 引用不应包含大括号:

The foo.bar reference should not contain the braces:

<p ng-hide="foo.bar">I could be shown, or I could be hidden</p>
<p ng-show="foo.bar">I could be shown, or I could be hidden</p>

Angular 表达式 需要在花括号绑定中，而 Angular 指令 没有.

Angular expressions need to be within the curly-brace bindings, where as Angular directives do not.

另请参阅了解 Angular 模板.