C++ Bison语法中的无用规则与非终结符

我有以下描述脚本语言语法的Bison文件：

%error-verbose
%{
#include "node.h"
NBlock *programBlock;

#define YYDEBUG 1

extern int yylex();
void yyerror(const char *) { printf("Error: %s\n", s); }
%}

%union {
    Node *node;
    NBlock *block;
    NBody *body;
    NHeader *header;
    NExpression *expression;
    NStatement *statement;
    NIdentifier *identifier;
    NVariableDeclaration *variableDeclaration;
    NDoWhileStatement *doWhileStatement;
    NWhileStatement *whileStatement;
    NIfStatement *ifStatement;
    NForStatement *forStatement;
    std::vector<NVariableDeclaration*> *variableDeclarations;
    std::vector<NExpression*> *expressions;
    std::vector<NStatement*> *statements;
    std:string *string;
    int token;
}


%token <string> TIDENTIFIER TINTEGER TDOUBLE
%token <token> TCEQ TCNE TCLT TCLE TCGT TCGE TEQUAL
%token <token> TLPAREN TRPAREN TLBRACE TRBRACE TCOMMA TDOT
%token <token> TPLUS TMINUS TMUL TDIV TDO TDOUBLE_TYPE TINT_TYPE
%token <token> TELSE TFOR TIF TSEMICOLON TTHEN TWHILE

%type <expression> expression term factor
%type <block> program body header tail statements
%type <statement> statement forStatement ifStatement doWhileStatement whileStatement variableDeclaration
%type <token> comparison
%type <string> identifier_type


%left TPLUS TMINUS
%left TMUL TDIV

%start program

%%

program:                TLBRACE body TRBRACE { printf("Reduce body to program\n"); }
                        ;

body:                   header TLBRACE block TRBRACE tail { printf("Reduce header block tail to body\n"); }
                        ;

header:                 TLBRACE variableDeclarations TRBRACE { printf("Reduce variableDeclarations to header\n"); }
                        | TLBRACE TRBRACE { printf("Reduce empty to header\n"); }
                        ;

variableDeclarations:   variableDeclaration TSEMICOLON { printf("Reduce variable declaration to header\n"); }
                        | variableDeclarations variableDeclaration TSEMICOLON { printf("Reduce variable declaration list to header\n"); }
                        ;

tail:                   TLBRACE statements TRBRACE { printf("reduce statement list to tail\n"); }
                        | TLBRACE TRBRACE { printf("Reduce empty to tal\n"); }
                        ;                   

statements:             statement TSEMICOLON { printf("Reduce statement to statement list\n"); }
                        | statements statement TSEMICOLON { printf("Reduce statement list to statement list\n"); }
                        ;

statement:              ifStatement { printf("Reduce if to statement\n"); };
                        | forStatement { printf("Reduce for to statement\n"); };
                        | doWhileStatement { printf("Reduce doWhile to statement\n"); };
                        | whileStatement { printf("reduce while to statement\n"); }
                        | expression { printf("Reduce expression to statement\n"); };
                        ;

forStatement:           TFOR TLPAREN expression TSEMICOLON expression TSEMICOLON expression TRPAREN block { printf("Reduce for to for statement\n"); }
                        ;


ifStatement:            TIF expression TTHEN block { printf("Reduce if to if statement\n"); }
                        | TIF expression block TELSE block { printf("Reduce ifelse to if statement\n"); }
                        ;

doWhileStatement:       TDO block TWHILE expression { printf("reduce dowhile to while statement\n"); }
                        ;

whileStatement:         TWHILE block expression { printf("Reduce while to while statement\n"); }
                        ;

block:                  TLBRACE statements TRBRACE { printf("Reduce statement list to block\n"); }
                        | TLBRACE TRBRACE { printf("Reduce empty to block\n"); }
                        ;

variableDeclaration:    identifier_type TIDENTIFIER { printf("reduce uninitialized identifier to variable declaration\n"); }
                        | identifier_type TIDENTIFIER TEQUAL expression { printf("Reduce initialized identifier to variable declaration\n"); }
                        ;

identifier_type:        TINT_TYPE { printf("Reduce int to identifier type\n"); }
                        | TDOUBLE_TYPE { printf("Reduce double to identifier type\n"); }
                        | { printf("Reduce empty to identifier type\n"); }
                        ;



expression:             TIDENTIFIER TEQUAL expression { printf("Reduce assignment to expression\n"); }
                        | TIDENTIFIER { printf("reduce identifier to expression\n"); }
                        | expression comparison expression { printf("Reduce comparison to expression\n"); }
                        | TLPAREN expression TRPAREN { printf("Reduce nested expression to expression\n"); }
                        | expression TPLUS term { printf("Reduce addition to expression\n"); }
                        | expression  TMINUS term { printf("Reduce subtraction to expression\n"); }
                        | term { printf("Reduce term to expresson\n"); }
                        | factor { printf("Reduce factor to expression\n"); }
                        ;

term:                   term TMUL factor { printf("Reduce multiplication to term\n"); }
                        | term TDIV factor { printf("Reduce division to term\n"); }
                        ;

factor:                 TIDENTIFIER { printf("Reduce identifier to factor\n"); }
                        | TINTEGER { printf("Reduce integer to numeric\n"); }
                        | TDOUBLE { printf("Reduce double to numeric\n"); }
                        ;

comparison:             TCEQ { printf("Reduce eq to comparison\n"); }
                        | TCNE { printf("Reduce ne to comparison\n"); }
                        | TCLT { printf("Reduce lt to comparison\n"); }
                        | TCLE { printf("Reduce le to comparison\n"); }
                        | TCGT { printf("reduce gt to comparison\n"); }
                        | TCGE { printf("Reduce ge to comparison\n"); }
                        | TPLUS { printf("Reduce plus to comparison\n"); }
                        | TMINUS { printf("Reduce minus to comparison\n"); }
                        | TMUL { printf("Reduce mul to comparison\n"); }
                        | TDIV { printf("Reduce div to comparison\n"); }
                        ; 

我得到以下错误：

$ bison -d -o parser.cpp parser.y
parser.y: attention: 1 nonterminal useless in grammar
parser.y: attention: 5 rules useless in grammar
parser.y:52.31-34: attention: nonterminal useless in grammar: term
parser.y:135.51-118: attention: rule useless in grammar: expression: expression TPLUS term
parser.y:136.51-123: attention: rule useless in grammar: expression: expression TMINUS term
parser.y:137.51-96: attention: rule useless in grammar: expression: term
parser.y:141.41-103: attention: rule useless in grammar: term: term TMUL factor
parser.y:142.51-107: attention: rule useless in grammar: term: term TDIV factor
parser.y: conflicts: 20 shift/reduce, 14 reduce/reduce
parser.y:145.41-96: attention: rule useless in parser due to conflicts: factor: TIDENTIFIER

---

我认为冲突在于解析器在找到TIDENTIFIER时不知道选择哪个规则。原因是

TIDENTIFIER

可以简化为

expression

因子

可以简化为

表达式

TIDENTIFIER

可以减少到

因子

因此，当解析器获得一个标识符时，它可以通过两种方式到达一个表达式

TIDENTIFIER -> expression  
TIDENTIFIER -> factor -> expression

我想在您的情况下，您可以安全地从factor中删除

TIDENTIFIER

，因为

factor

已减少为仅

表达式

然而，一种更合适的方法（至少对于大多数通用语言而言）可能是将

TIDENTIFIER

保留在

factor

中，并在

语句中处理其余部分。例如，
TIDENTIFIER赋值表达式
可以是有效的
语句

顺便说一句，您的
语句
包括
表达式
。这肯定不是你想要的<代码>语句

应始终与表达式不同（毕竟，这就是为什么需要两条规则的原因）

---

我想你应该检查一下这样的案子。如果您有一个只使用另一个规则（没有任何其他标记或其他规则）的规则，那么它可能是错误的。从某种意义上说，这不是你想要的。

我认为无用的规则/非终结符只是你冲突的结果。一旦你把它们修好，我希望它们会消失。看起来至少需要定义更多的运算符优先级。我修复了无用的规则和非终结符。我仍然需要在一个状态下修复8个班次/减少冲突。无论如何，我还是要问另一个不同的问题。星尘坦克_

TIDENTIFIER -> expression  
TIDENTIFIER -> factor -> expression