约束可以是简单的单值，也可以是复合值或多个类型。

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

youdao

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。

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

youdao

第二种操作符是针对组成数据元素而非整个复合值的一种操作。

A second kind of operator is one that ACTS on component data elements instead of on the entire composite value.

youdao

The square brackets here, meanwhile, say this is a special type of variable that's got multiple values inside of it.

这里的这个方括号，同时，说明这个是特殊的变量类型,里面有很多复合的值。

哈佛公开课 - 计算机科学课程节选