如何声明属于undef或角色的属性？

Question

我有一个Moose类，其属性具有特定的作用：

has foo => (is => 'rw', does => 'Foo::Bar');

现在如何更改声明以允许undef？我尝试了does => 'Maybe[Foo::Bar]'，但这似乎不起作用，不通过合法的定义值。

Answer 1

可以使用isa指定此类约束。根据{{​​3}}角色名称也是有效选项。

package Foo::Bar;
use Moose::Role;


package WithFooBar;
use Moose;
with 'Foo::Bar';


package Thing;
use Moose;

has foo => (is => 'ro', isa => 'Maybe[Foo::Bar]');

package main;
use Test::More;

ok(Thing->new(foo => undef));               # pass
ok(Thing->new(foo => WithFooBar->new));     # pass
ok(Thing->new(foo => Thing->new));          # fails

done_testing;