为了方便在模板中判断变量是否存在，Blade提供了@isset指令，用法如下：
@isset($foo)
@else
@endisset实现如下：
//src/Illuminate/View/Compilers/Concerns/CompilesConditionals.php
/**
* Compile the if-isset statements into valid PHP.
*
* @param string $expression
* @return string
*/
protected function compileIsset($expression)
{
return "<?php if(isset{$expression}): ?>";
}
/**
* Compile the end-isset statements into valid PHP.
*
* @return string
*/
protected function compileEndIsset()
{
return '<?php endif; ?>';
}