ANTLR 语法互左递归

问题描述

我知道这个问题已经被问过很多次了.我正在尝试使用 ANTLR 构建语法.

I do know that this question has been asked a lot of times. I am trying to build a grammar using ANTLR.

Predicate           : LOWERCASE | Predicate VarChars ;

VarChars            : LOWERCASE | UPPERCASE;

fragment LOWERCASE  : [a-z] ;

fragment UPPERCASE  : [A-Z] ;

我收到以下错误:以下规则集相互左递归[谓词]"

I am getting the following error :"The following sets of rules are mutually left-recursive [Predicate]"

请告诉我这是如何解决的.如何删除我的 antlr 语法中的相互左递归.

Please show me how this is fixed. How to remove the mutual left recursion in my antlr grammar.

推荐答案

彻底摆脱Predicate"的递归出现.单独的 VarChars 就足以表示小写或大写.使用 + 后缀表示一个或多个实例":

Get rid of the recursive occurrence of "Predicate" altogether. VarChars alone is sufficient to mean lowercase or uppercase. Use the + suffix to indicate "one or more instances":

fragment LOWERCASE : [a-z];
fragment UPPERCASE : [A-Z];

VARCHARS  : LOWERCASE | UPPERCASE;
PREDICATE : LOWERCASE VARCHARS+;

应用于您的示例，这将取消p"和PA"作为谓词的资格，并限制pA".

Applied to your examples, this would disqualify "p" and "PA" as predicates, and qualify "pA".

观察 PREDICATE 和 VARCHARS 如何仍然是词法规则(与句法规则相对)，因为它们描述了词素是如何形成的.因此它遵循全大写命名约定(Antlr 不在乎，但它提高了可读性).

Observe how PREDICATE and VARCHARS are still lexer rules (as opposed to syntactical rules) as they describe how a lexeme is formed. Therefore it follows the all-uppercase naming convention (Antlr does not care, but it improves readability).

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