Question

我正在尝试用野牛构建一个解析器，并将我的所有错误缩小到一个难度。

以下是bison的调试输出，其中包含错误所在的状态：

state 120

   12 statement_list: statement_list . SEMICOLON statement
   24 if_statement: IF conditional THEN statement_lists ELSE statement_list .

    SEMICOLON  shift, and go to state 50

    SEMICOLON  [reduce using rule 24 (if_statement)]
    $default   reduce using rule 24 (if_statement)

以下是parser.y source

中的翻译规则

%%

program              : ID COLON block ENDP ID POINT
                     ;
block                : CODE statement_list
                     | DECLARATIONS declaration_block CODE statement_list
                     ;
declaration_block    : id_list OF TYPE type SEMICOLON
                     | declaration_block id_list OF TYPE type SEMICOLON
                     ;
id_list              : ID
                     | ID COMMA id_list
                     ;
type                 : CHARACTER
                     | INTEGER
                     | REAL
                     ;
statement_list       : statement
                     | statement_list SEMICOLON statement
                     ;
statement_lists      : statement
                     | statement_list SEMICOLON statement
                     ;
statement            : assignment_statement
                     | if_statement
                     | do_statement
                     | while_statement
                     | for_statement
                     | write_statement
                     | read_statement
                     ;
assignment_statement : expression OUTPUTTO ID
                     ;
if_statement         : IF conditional THEN statement_lists ENDIF
                     | IF conditional THEN statement_lists ELSE statement_list
                     ;
do_statement         : DO statement_list WHILE conditional ENDDO
                     ;
while_statement      : WHILE conditional DO statement_list ENDWHILE
                     ;
for_statement        : FOR ID IS expression BY expressions TO expression DO statement_list ENDFOR
                     ;
write_statement      : WRITE BRA output_list KET
                     | NEWLINE
                     ;
read_statement       : READ BRA ID KET
                     ;
output_list          : value
                     | value COMMA output_list
                     ;
condition            : expression comparator expression
                     ;
conditional          : condition
                     | NOT conditional
                     | condition AND conditional
                     | condition OR conditional
                     ;
comparator           : ASSIGNMENT
                     | BETWEEN
                     | LT
                     | GT
                     | LESSEQUAL
                     | GREATEREQUAL
                     ;
expression           : term
                     | term PLUS expression
                     | term MINUS expression
                     ;
expressions          : term
                     | term PLUS expressions
                     | term MINUS expressions
                     ;
term                 : value
                     | value MULTIPLY term
                     | value DIVIDE term
                     ;
value                : ID
                     | constant
                     | BRA expression KET
                     ;
constant             : number_constant
                     | CHARCONST
                     ;
number_constant      : NUMBER
                     | MINUS NUMBER
                     | NUMBER POINT NUMBER
                     | MINUS NUMBER POINT NUMBER
                     ;              
%%

当我删除if_statement规则时没有错误，所以我大大缩小了它，但仍然无法解决错误。

感谢您的帮助。

Answer 1

考虑这个陈述：如果条件则s2否则s3; S4

有两种解释：

if condition then
   s1;
else 
   s2;
s3;

另一个是：

if condition then
   s1;
else
   s2;
   s3;

在第一个中，法规列表由if语句和s3组成。而另一个语句只由一个if语句组成。这就是歧义的来源。当存在shift-reduce冲突时，Bison会选择shift to reduce，因此在上述情况下，解析器将选择转换s3。

由于ENDIF语句中有if-then，请考虑在ENDIF语句中引入if-then-else，然后问题就解决了。

Answer 2

我认为您在ENDIF规则中缺少IF-THEN-ELSE-ENDIF。

野牛：冲突：1班/减少错误

2 个答案: