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 我不认为你想要什么可以用UpValues （ UpValues ）来完成，因为符号（标签）必须不比第一级更深才能定义工作。 此外，Mathematica中所需的语义有点不寻常，因为大多数Mathematica表达式是不可变的（不是L值），并且它们的部分不能被赋值。 我相信这段代码会做类似于你想要的东西： 
 
Unprotect[Set];
Set[var_Symbol, rhs_] /; 
   MatchQ[Hold[var] /. OwnValues[var], Hold[_struct]] := Set[var[[1]], rhs];
Protect[Set];
 
 例如： 
 
In[33]:= var1 = struct[{1, 2}]

Out[33]= struct[{1, 2}]

In[34]:= var1 = "Success!"

Out[34]= "Success!"

In[35]:= var1

Out[35]= struct["Success!"]
 
 但通常，不建议将DownValues添加到Set等重要命令，因为这可能会以微妙的方式破坏系统。 
 
 编辑 
 
 扩展一下你的尝试失败的原因：Mathematica使用参数保持机制（ Hold* - 属性， 在此描述）实现流控制和赋值运算符。 该机制特别允许它模仿任务所需的传递引用语义。 但是，在你分配给var1 ， Set不知道var1已经存储了什么，因为它只有符号var1 ，而不是它的值。 模式_struct不匹配，因为即使变量已经存储了一些struct ， Set只有变量名。 为了匹配成功， Set内的变量将不得不评估为其值。 但是，那么价值是不变的，你不能指定它。 我建议的代码测试变量是否具有struct[something]形式的赋值，如果是，则修改第一部分（ Part命令是一个例外，它可以修改L值表达式的一部分，前提是那些部分已经存在）。 
 
 您可以在许多地方阅读关于Hold* - 属性和相关问题的更多信息，例如这里和这里 

I don't think that what you want can be done with UpValues (alas), since the symbol (tag) must be not deeper than level one for definition to work. Also, the semantics you want is somewhat unusual in Mathematica, since most Mathematica expressions are immutable (not L-values), and their parts can not be assigned values. I believe that this code will do something similar to what you want:
 
Unprotect[Set];
Set[var_Symbol, rhs_] /; 
   MatchQ[Hold[var] /. OwnValues[var], Hold[_struct]] := Set[var[[1]], rhs];
Protect[Set];
 
For example:
 
In[33]:= var1 = struct[{1, 2}]

Out[33]= struct[{1, 2}]

In[34]:= var1 = "Success!"

Out[34]= "Success!"

In[35]:= var1

Out[35]= struct["Success!"]
 
But generally, adding DownValues to such important commands as Set is not recommended since this may corrupt the system in subtle ways.
 
EDIT
 
Expanding a bit on why your attempt failed: Mathematica implements flow control and assignment operators using the mechanism of argument holding (Hold* - attributes, described here). This mechanism allows it to, in particular, imitate pass-by-reference semantics needed for assignments. But then, at the moment when you assign to var1, Set does not know what is stored in var1 already, since it only has the symbol var1, not its value. The pattern _struct does not match because, even if the variable already stores some struct, Set only has the variable name. For the match to be successful, the variable inside Set would have to evaluate to its value. But then, the value is immutable and you can not assign to it. The code I suggested tests whether the variable has an assigned value that is of the form struct[something], and if so, modifies the first part (the Part command is an exception, it can modify parts of an L-value expression provided that those parts already exist).
 
You can read more on the topics of Hold* - attributes and related issues in many places, for example here and here