表达式的语法不允许外括号 [英] Grammar for expressions which disallows outer parentheses

问题描述

$ $ $ $ $ $ $ $ $ $ code> expression：expression BITWISE_OR xor_expression
| xor_expression
xor_expression：xor_expression BITWISE_XOR and_expression
| and_expression
and_expression：and_expression BITWISE_AND shift_expression
| shift_expression
shift_expression：shift_expression LEFT_SHIFT arith_expression
| shift_expression RIGHT_SHIFT arith_expression
| arith_expression
arith_expression：arith_expression PLUS项
| arith_expression MINUS项
|期限
期限：期限TIMES因子
|期限DIVIDE因子
|因子
因子：NUMBER
| LPAREN表达式RPAREN

这似乎工作正常，但不符合我的需要，因为它允许外部括号例如（（3 + 4）* 2）。

如何更改语法以禁止外括号，而仍然允许他们在表达中，例如（3 + 4）* 2 ，甚至冗余例如（3 * 4）+ 2 ？

解决方案

将此规则添加到你的语法：

  top_level：expression BITWISE_OR xor_expression 
 | xor_expression BITWISE_XOR and_expression 
 | and_expression BITWISE_AND shift_expression 
 | shift_expression LEFT_SHIFT arith_expression 
 | shift_expression RIGHT_SHIFT arith_expression 
 | arith_expression术语
 | arith_expression MINUS项
 |期限TIMES因子
 |期限DIVIDE因子
 | NUMBER 
  

，并使用top_level，您想要没有外括号的表达式。

I have the following grammar for expressions involving binary operators (| ^ & << >> + - * /):

expression       : expression BITWISE_OR xor_expression
                 | xor_expression
xor_expression   : xor_expression BITWISE_XOR and_expression
                 | and_expression
and_expression   : and_expression BITWISE_AND shift_expression
                 | shift_expression
shift_expression : shift_expression LEFT_SHIFT arith_expression
                 | shift_expression RIGHT_SHIFT arith_expression
                 | arith_expression
arith_expression : arith_expression PLUS term
                 | arith_expression MINUS term
                 | term
term             : term TIMES factor
                 | term DIVIDE factor
                 | factor
factor           : NUMBER
                 | LPAREN expression RPAREN

This seems to work fine, but doesn't quite match my needs because it allows outer parentheses e.g. ((3 + 4) * 2).

How can I change the grammar to disallow outer parentheses, while still allowing them within expressions e.g. (3 + 4) * 2, even redundantly e.g. (3 * 4) + 2?

解决方案

Add this rule to your grammar:

top_level : expression BITWISE_OR xor_expression
          | xor_expression BITWISE_XOR and_expression
          | and_expression BITWISE_AND shift_expression
          | shift_expression LEFT_SHIFT arith_expression
          | shift_expression RIGHT_SHIFT arith_expression
          | arith_expression PLUS term
          | arith_expression MINUS term
          | term TIMES factor
          | term DIVIDE factor
          | NUMBER

and use top_level where you want expressions without outer parens.

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