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我有以下 Bison 文件来描述我的脚本语言语法：

%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"); }
                        ;

我真的不明白为什么我会收到与无用非终结符和无用规则相关的错误。

运行命令：

bison -d -o parser.cpp parser.y

我收到以下错误：

$ 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
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1 回答 1

2

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

TIDENTIFIER可以减少到expression
factor可以减少到expression
TIDENTIFIER可以减少到factor

因此，当解析器获得标识符时，它可以通过两种方式到达表达式。

TIDENTIFIER -> expression  
TIDENTIFIER -> factor -> expression

在您的情况下，我认为您可以安全地TIDENTIFIER从因素中删除，因为无论如何factor都会减少到ONLY expression。

然而，一种更合适的方法（至少对于大多数常用语言而言）可能是保留TIDENTIFIER并factor处理statement. 例如TIDENTIFIER ASSIGN expression可以是一个有效的statement.

顺便说一句，您的statement包括expression. 这可能不是你想要的。statement应该总是不同于expression（毕竟这就是为什么你需要两个规则）。

我认为你应该检查这样的情况。如果您的规则仅使用另一个规则（没有任何其他标记或其他规则），那么它可能是错误的。从某种意义上说错了，这不是你想要的。

于 2013-03-27T14:54:57.780 回答