ANTLR:空状态不起作用 [英] ANTLR: empty condition not working

问题描述

我希望能够解析 int [] 或 int 标记.

I want to be able to parse int [] or int tokens.

请考虑以下语法:

TYPE    :   'int' AFTERINT;
AFTERINT:   '[' ']';

当然可以，但是仅适用于 int [] .为了使其也适用于 int ，我将AFTERINT更改为此(添加了一个空条件':

Of course it works, but only for int []. To make it work for int too, I changed AFTERINT to this (added an empty condition':

AFTERINT:   '[' ']' |
              |;

但是现在我得到了这个警告和错误:

But now I get this warning and error:

[13:34:08]警告(200):MiniJava.g:5:9:决策可以匹配输入 例如"使用多种替代方式:2，3

[13:34:08] warning(200): MiniJava.g:5:9: Decision can match input such as "" using multiple alternatives: 2, 3

结果，该输入的替代项3被禁用[13:34:08] 错误(201):MiniJava.g:5:9:永远不可能有以下替代方案 匹配:3

As a result, alternative(s) 3 were disabled for that input [13:34:08] error(201): MiniJava.g:5:9: The following alternatives can never be matched: 3

为什么空条件不起作用?

Why won't empty condition work?

推荐答案

词法分析器无法处理与空字符串匹配的标记.如果您考虑片刻，这并不奇怪:毕竟，您的输入中有无数的空字符串.该词法分析器始终会生成一个空字符串作为有效令牌，从而导致无限循环.

The lexer cannot cope with tokens that match empty string. If you think about it for a moment, this is not surprising: after all, there are an infinite amount of empty strings in your input. The lexer would always produce an empty string as a valid token, resulting in an infinite loop.

类型识别不属于词法分析器，而是属于解析器:

The recognition of types does not belong in the lexer, but in the parser:

type
 : (INT | DOUBLE | BOOLEAN | ID) (OBR CBR)?
 ;

OBR     : '[';
CBR     : ']';
INT     : 'int';
DOUBLE  : 'double';
BOOLEAN : 'boolean';
ID      : ('a'..'z' | 'A'..'Z')+;

每当您开始组合不同的 type 字符以创建(单个)令牌时，通常最好为此创建一个解析器规则.将词法分析器规则(标记)视为语言的最小组成部分.从这些构建块中，您组成解析器规则.

Whenever you start combining different type of characters to create a (single) token, it's usually better to create a parser rule for this. Think of lexer rules (tokens) as the smallest building block of your language. From these building blocks, you compose parser rules.

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