为什么变量名中不能包含空格? [英] Why can't variable names have spaces in them?
问题描述
变量名中不允许使用空格还是出于惯例?
Is there a technical reason why spaces aren't allowed in variable names or is it down to convention?
例如，是什么阻止我们做这样的事情?:
For example, what's stopping us from doing something like this?:
average score = sum of scores / number of scores
想到的唯一一个问题是关键字，但是可以简单地将它们限制在变量名中使用，而词法分析器将能够区分变量的一部分和关键字.
The only issue that comes to mind is keywords, but one could simply restrict the use of them in a variable name, and the lexer would be able to distinguish between part of a variable and a keyword.
推荐答案
除了语言设计者的决定和单令牌标识符的历史之外，没有根本的原因.实际上，某些语言确实允许使用多令牌标识符:MultiMedia Fusion的表达语言，一些Mac电子表格/笔记本软件，其名称使我不知所措，我敢肯定还有其他语言.尽管如此，有几个考虑因素使这个问题变得微不足道.
There’s no fundamental reason, apart from the decisions of language designers and a history of single-token identifiers. Some languages in fact do allow multi-token identifiers: MultiMedia Fusion’s expression language, some Mac spreadsheet/notebook software whose name escapes me, and I’m sure of others. There are several considerations that make the problem nontrivial, though.
假设语言是自由格式，则需要规范的表示形式，以便像account name这样的标识符被视为相同，而与空白无关.编译器可能需要使用一些处理约定来取悦链接器.然后，您必须考虑它对外贸出口的影响-为什么C ++具有extern "C"链接说明符来禁用重整.
Presuming the language is free-form, you need a canonical representation, so that an identifier like account name is treated the same regardless of whitespace. A compiler would probably need to use some mangling convention to please a linker. Then you have to consider the effect of that on foreign exports—why C++ has the extern "C" linkage specifier to disable mangling.
正如您所见，关键字是一个问题.大多数C系列语言都具有与标识符不同的词法类关键字，这些标识符与上下文无关.您不能在C ++中命名变量class.这可以通过禁止多令牌标识符中的关键字来解决:
Keywords are an issue, as you have seen. Most C-family languages have a lexical class of keywords distinct from identifiers, which are not context-sensitive. You cannot name a variable class in C++. This can be solved by disallowing keywords in multi-token identifiers:
if account age < 13 then child account = true;
在这里，if和then不能是标识符的一部分，因此与account age和child account毫无歧义.或者，您可以在任何地方都要求标点符号:
Here, if and then cannot be part of an identifier, so there is no ambiguity with account age and child account. Alternatively, you can require punctuation everywhere:
if (account age < 13) {
child account = true;
}
最后一个选择是使关键字普遍与上下文相关，从而导致诸如以下的怪异现象:
The last option is to make keywords pervasively context-sensitive, leading to such monstrosities as:
IF IF = THEN THEN ELSE = THEN ELSE THEN = ELSE
最大的问题是，并置是一种非常强大的句法结构，您不想轻易占据它.允许多令牌标识符可防止将并置用于其他目的，例如功能应用程序或组合.我认为，最好允许大多数 nonwhitespace 字符，从而允许使用canonical-venomous-frobnicator这样的标识符.仍然可读性强，但产生歧义的机会较少.
The biggest issue is that juxtaposition is an extremely powerful syntactic construct, and you don’t want to occupy it lightly. Allowing multi-token identifiers prevents using juxtaposition for another purpose, such as function application or composition. Far better, I think, just to allow most nonwhitespace characters and thereby permit such identifiers as canonical-venomous-frobnicator. Still plenty readable but with fewer opportunities for ambiguity.
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