如何在ANTLR4中有时将关键字解析为普通单词 [英] How to parse keywords as normal words some of the time in ANTLR4

问题描述

我有一种语言，其中包含诸如hello之类的关键字，它们只是某些类型的句子中的关键字.例如，在其他类型的句子中，这些单词应作为ID进行匹配.这是一个讲述故事的超简单语法:

I have a language with keywords like hello that are only keywords in certain types of sentences. In other types of sentences, these words should be matched as an ID, for example. Here's a super simple grammar that tells the story:

grammar Hello;

file : ( sentence )* ;
sentence : 'hello' ID PERIOD
         | INT ID PERIOD;

ID  : [a-z]+ ;
INT : [0-9]+ ;
WS  : [ \t\r\n]+ -> skip ;
PERIOD : '.' ;

我希望这些句子有效:

hello fred.
31 cheeseburgers.
6 hello.

但是最后一句话在该语法中不起作用.单词hello是类型hello的令牌，而不是类型ID的令牌.词法分析器似乎抓住了所有的问候，并将它们转换为该类型的标记.

but that last sentence doesn't work in this grammar. The word hello is a token of type hello and not of type ID. It seems like the lexer grabs all the hellos and turns them into tokens of that type.

这是一种疯狂的方式来解释我想要的东西:

Here's a crazy way to do it, to explain what I want:

sentence : 'hello' ID PERIOD
         | INT crazyID PERIOD;

crazyID : ID | 'hello' ;

但是用我的真实语言来说，有很多像hello这样的关键字要处理，所以，是的，这似乎很疯狂.

but in my real language, there are a lot of keywords like hello to deal with, so, yeah, that way seems crazy.

是否有一种合理，紧凑，与目标语言无关的方式来处理此问题?

Is there a reasonable, compact, target-language-independent way to handle this?

推荐答案

处理关键字的标准方法:

A standard way of handling keywords:

file     : ( sentence )* EOF ;
sentence : key=( KEYWORD | INT ) id=( KEYWORD | ID ) PERIOD ;

KEYWORD : 'hello' | 'goodbye' ; // list others as alts
PERIOD  : '.' ;
ID      : [a-z]+ ;
INT     : [0-9]+ ;
WS      : [ \t\r\n]+ -> skip ;

根据在ID规则之前列出的KEYWORD规则，可以解决KEYWORD和ID规则之间的模棱两可的情况.

The seeming ambiguity between the KEYWORD and ID rules is resolved based on the KEYWORD rule being listed before the ID rule.

在解析器SentenceContext中，将生成TerminalNode变量key和id，并且在解析时将有效地保存匹配的标记，从而轻松进行位置识别.

In the parser SentenceContext, TerminalNode variables key and id will be generated and, on parsing, will effectively hold the matched tokens, allowing easy positional identification.

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