[英]Optional vs. mandatory terminators in context-free grammar definition
In a book chapter about compilers, there's the following grammar definition and example code. 在关于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;
}
}
How is the code's inner-most while
loop valid without {
}
? 如何为代码的最内侧
while
循环有效,而不{
}
? It looks to me that the statement definition requires them. 在我看来,语句定义需要它们。 Is this a mistake in the book or am I misunderstanding the syntax?
这是书中的错误还是我误解了语法?
[Edit] My apologies for any ambiguity. [编辑]我对任何含糊不清表示歉意。 Everything typed above is verbatim from the book.
上面输入的所有内容都是从书中逐字记录下来的。 The omissions were not my doing.
遗漏不是我的行为。
Your while statement says that after the )
comes a statement. 你同时声明说,以后
)
来声明。 Your grammar doesn't fully specify statement
, but it doesn't require braces. 您的语法没有完全指定
statement
,但它不需要大括号。 Braces are only needed for a statement sequence. 只有语句序列才需要大括号。
Consider your example code again: 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 }
Why are you concerned that line 6 lacks braces, but don't care that lines 2, 3, and 7 are missing them too? 你为什么担心第6行没有括号,但是不在乎第2,3和7行也缺少它们? 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
. 语法是说
while
循环以statement
结束,而statementSequence
及其必需的大括号只是statement
的许多替代方案之一。 Lines 5 and 6 match that rule exactly—except for the ';'
第5行和第6行完全匹配该规则 - 除了
';'
, which doesn't have a place in the rule. ,在规则中没有位置。
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