Constraints can be simple single values, or they can be a compound value or multiple types.

约束可以是简单的单值，也可以是复合值或多个类型。

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Instead of checking for a default compound value, you can also catch the access to an undefined variable. E.g.

不用再检查默认的复合值，您也可以捕获对未定义变量的访问。

youdao

Instead of checking for a default compound value, you can also catch the access to an undefined variable. E.g.

不用再检查默认的复合值，您也可以捕获对未定义变量的访问。

youdao