上下文无关语法定义中的可选终止符与强制终止符

发布于 2024-08-03 18:37:31 字数 839 浏览 2 评论 0原文

在有关编译器的书籍章节中，有以下语法定义和示例代码。

...
statement: whileStatement
           | ifStatement
           | ... // Other statement possibilities
           | '{' statementSequence '}'
whileStatement: 'while' '(' expression ')' statement
ifStatement: ... // Definition of "if"
statementSequence: '' // empty sequence (null)
                   | statement ';' statementSequence
expression: ... // Definition of "expression"
...             // More definitions follow

while (expression) {
 statement;
 statement;
 while (expression) {
  while(expression)
     statement;
  statement;
 }
}

如果没有 { }，代码最内部的 while 循环如何有效？在我看来，语句定义需要它们。这是书中的错误还是我误解了语法？

[编辑] 对于任何含糊之处，我深表歉意。上面输入的所有内容都是逐字逐句摘自书中的。这些遗漏不是我造成的。

原文

In a book chapter about compilers, there's the following grammar definition and example code.

...
statement: whileStatement
           | ifStatement
           | ... // Other statement possibilities
           | '{' statementSequence '}'
whileStatement: 'while' '(' expression ')' statement
ifStatement: ... // Definition of "if"
statementSequence: '' // empty sequence (null)
                   | statement ';' statementSequence
expression: ... // Definition of "expression"
...             // More definitions follow

while (expression) {
 statement;
 statement;
 while (expression) {
  while(expression)
     statement;
  statement;
 }
}

How is the code's inner-most while loop valid without { }? It looks to me that the statement definition requires them. Is this a mistake in the book or am I misunderstanding the syntax?

[Edit] My apologies for any ambiguity. Everything typed above is verbatim from the book. The omissions were not my doing.

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み青杉依旧 2024-08-10 18:37:31

再次考虑您的示例代码：

1 while (expression) {
2  statement;
3  statement;
4  while (expression) {
5   while(expression)
6      statement;
7   statement;
8  }
9 }

为什么您担心第 6 行缺少大括号，但不关心第 2、3 和 7 行也缺少大括号？语法说 while 循环以 statement 结束，而 statementSequence 及其所需的大括号只是许多替代方案之一一个声明。第 5 行和第 6 行与该规则完全匹配，但 ';' 除外，它在规则中没有位置。

Consider your example code again:

1 while (expression) {
2  statement;
3  statement;
4  while (expression) {
5   while(expression)
6      statement;
7   statement;
8  }
9 }

Why are you concerned that line 6 lacks braces, but don't care that lines 2, 3, and 7 are missing them too? The grammar is saying that a while loop ends with a statement, and a statementSequence, with its required braces, is just one of many alternatives for a statement. Lines 5 and 6 match that rule exactly—except for the ';', which doesn't have a place in the rule.

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