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建立野牛语法时，终端上显示了太多无用的规则

[英]too many useless rules show up on terminal while building a bison grammar

 原文  2017-04-30 23:40:39  1   1    c/ grammar/ bison/ compiler-warnings

问题描述

编码： 

%{
#include <stdio.h>
#include <string.h>
#include <malloc.h>
#include "gener.h"
#include "sym_tab.h"
#include "scope.h"
#include "defs.h"
extern char *yytext;
#define YYPRINT(file, type, value)   yyprint (file, type, value)
%}

%union
{
char *name;
char idtype;
decl_list idlist;
/*decl_list is in gener.h,  
  you should #include "gener.h"
*/
}

%token PROGRAM
%token IDENTIFIER 
%token DECIMAL_CONSTANT
%token DECLARE ENDDECLARE

%token AND CALL DEFAULT
%token FUNCTION PROCEDURE IN INOUT
%token IF ELSE DO WHILE FOR OR NOT SELECT
%token RETURN EXIT  PRINT 
%token EQ_LT EQ_GT NE EXACT 
%left '+' '-'
%right '*' '/'



%start program
%%

program
        : %empty
        | PROGRAM IDENTIFIER block
        ;

block
        : "{" declarations subprograms sequence "}"
        ;
declarations
        : %empty
        | DECLARE varlist ENDDECLARE 
        ;
varlist
        : %empty
        | assignment_stat identifier2
        ;

identifier2 : ',' assignment_stat
        | ',' assignment_stat 

        ;
subprograms 
        : func
        | subprograms ','  
        ;
func        
        : PROCEDURE IDENTIFIER funcbody
        | FUNCTION IDENTIFIER funcbody
        ;
funcbody    
        : formalpars block
        ;
formalpars  
        : '('')'
        | '(' formalparlist ')'

        ;
formalparlist   
        : formalparitem
        | formalparlist ','

        ;
formalparitem   
        : IN IDENTIFIER
        | INOUT IDENTIFIER
        ;       
sequence
        : statement statement_sequence
        ;
statement_sequence
        : ";"statement
        | ';' statement statement_sequence
        ;
brackets_seq    
        : '{' sequence '}'
        ;
brack_or_stat   
        : brackets_seq
        | statement
        ;


statement   
        : %empty
        | assignment_stat
        | if_stat
        | do_while_stat
        | while_stat
        | select_stat
        | exit_stat
        | return_stat
        | print_stat
        | call_stat
        ;
assignment_stat
            : IDENTIFIER EXACT IDENTIFIER 
            ;

if_stat
        : IF '(' condition')' brack_or_stat elsepart
        ;
elsepart
        : %empty
        | ELSE brack_or_stat
        ;
while_stat
        : WHILE '(' condition')' brack_or_stat
        ;
select_stat
        : SELECT'(' IDENTIFIER')'
          dec 
          DEFAULT":" brack_or_stat
        ;
dec
        : DECIMAL_CONSTANT ':' brack_or_stat
        | dec ','
        ;
do_while_stat
        : DO brack_or_stat WHILE '(' condition ')'
        ;
exit_stat   
        : EXIT
        ;
return_stat
        : RETURN '(' expression ')'
        ;
print_stat
        : PRINT '('expression ')'
        ;
call_stat
        : CALL IDENTIFIER actualpars
        ;
actualpars
        : '('')'
        | '('actualparlist')'
        ;
actualparlist
        : actualparitem 
        | actualparlist ','
        ;
actualparitem
        : IN expression
        | INOUT IDENTIFIER
        ;
condition
        : boolterm or_boolterm 
        ;
or_boolterm 
        : OR boolterm
        | OR boolterm or_boolterm
        ;
boolterm    
        : boolfactor and_boolfactor
        ;
and_boolfactor
        : AND boolfactor
        | AND boolfactor and_boolfactor
        ;
boolfactor  
        : NOT '[' condition ']'
        | '[' condition ']'
        | expression relational_oper expression
        ;
expression
        : optional_sign term add_oper_term
        ;
add_oper_term
        : add_oper term
        | add_oper term add_oper_term
        ;
term        
        : factor mul_oper_factor
        ;
mul_oper_factor
        : mul_oper factor 
        | mul_oper factor mul_oper_factor
        ;
factor
        : DECIMAL_CONSTANT
        | expression
        | IDENTIFIER idtail
        ;
idtail
        : %empty
        | actualpars
        ;
relational_oper
        : '='
        | '<'
        | "<="
        | "<>"
        | ">="
        | '>'
        ;
add_oper
        : '+'
        | '-'
        ;
mul_oper
        : '*'
        | '/'
        ;
optional_sign
        : add_oper
        | %empty
        ;



%%

这是我为野牛创建的语法，但是当我在终端上尝试野​​牛gram.y时，会出现一堆警告。 试图切换他们被写的顺序，但我仍然得到同样的东西这里是终端输出 

gram.y: warning: 44 nonterminals useless in grammar [-Wother]
gram.y: warning: 86 rules useless in grammar [-Wother]
gram.y:43.38-42: warning: nonterminal useless in grammar: block [-Wother]
        | PROGRAM IDENTIFIER block
                                      ^^^^^
gram.y:47.23-34: warning: nonterminal useless in grammar: declarations [-Wother]
        : "{" declarations subprograms sequence "}"
                       ^^^^^^^^^^^^
gram.y:51.27-33: warning: nonterminal useless in grammar: varlist [-Wother]
        | DECLARE varlist ENDDECLARE 
                           ^^^^^^^
gram.y:55.35-45: warning: nonterminal useless in grammar: identifier2 [-Wother]
        | assignment_stat identifier2
                                   ^^^^^^^^^^^
gram.y:47.36-46: warning: nonterminal useless in grammar: subprograms [-Wother]
        : "{" declarations subprograms sequence "}"
                                    ^^^^^^^^^^^
gram.y:63.19-22: warning: nonterminal useless in grammar: func [-Wother]
        : func
                   ^^^^
gram.y:67.40-47: warning: nonterminal useless in grammar: funcbody [-Wother]
        : PROCEDURE IDENTIFIER funcbody
                                        ^^^^^^^^
gram.y:71.19-28: warning: nonterminal useless in grammar: formalpars [-Wother]
        : formalpars block
                   ^^^^^^^^^^
gram.y:75.23-35: warning: nonterminal useless in grammar: formalparlist [-Wother]
        | '(' formalparlist ')'
                       ^^^^^^^^^^^^^
gram.y:79.19-31: warning: nonterminal useless in grammar: formalparitem [-Wother]
        : formalparitem
                   ^^^^^^^^^^^^^
gram.y:47.48-55: warning: nonterminal useless in grammar: sequence [-Wother]
        : "{" declarations subprograms sequence "}"
                                                ^^^^^^^^
gram.y:88.29-46: warning: nonterminal useless in grammar: statement_sequence [-Wother]
        : statement statement_sequence
                             ^^^^^^^^^^^^^^^^^^
gram.y:94.1-12: warning: nonterminal useless in grammar: brackets_seq [-Wother]
 brackets_seq   
 ^^^^^^^^^^^^
gram.y:97.1-13: warning: nonterminal useless in grammar: brack_or_stat [-Wother]
 brack_or_stat  
 ^^^^^^^^^^^^^
gram.y:88.19-27: warning: nonterminal useless in grammar: statement [-Wother]
        : statement statement_sequence
                   ^^^^^^^^^
gram.y:55.19-33: warning: nonterminal useless in grammar: assignment_stat [-Wother]
        | assignment_stat identifier2
                   ^^^^^^^^^^^^^^^
gram.y:106.19-25: warning: nonterminal useless in grammar: if_stat [-Wother]
        | if_stat
                   ^^^^^^^
gram.y:120.53-60: warning: nonterminal useless in grammar: elsepart [-Wother]
        : IF '(' condition')' brack_or_stat elsepart
                                                     ^^^^^^^^
gram.y:108.19-28: warning: nonterminal useless in grammar: while_stat [-Wother]
        | while_stat
                   ^^^^^^^^^^
gram.y:109.19-29: warning: nonterminal useless in grammar: select_stat [-Wother]
        | select_stat
                   ^^^^^^^^^^^
gram.y:131.19-21: warning: nonterminal useless in grammar: dec [-Wother]
          dec 
                   ^^^
gram.y:107.19-31: warning: nonterminal useless in grammar: do_while_stat [-Wother]
        | do_while_stat
                   ^^^^^^^^^^^^^
gram.y:110.19-27: warning: nonterminal useless in grammar: exit_stat [-Wother]
        | exit_stat
                   ^^^^^^^^^
gram.y:111.19-29: warning: nonterminal useless in grammar: return_stat [-Wother]
        | return_stat
                   ^^^^^^^^^^^
gram.y:112.19-28: warning: nonterminal useless in grammar: print_stat [-Wother]
        | print_stat
                   ^^^^^^^^^^
gram.y:113.19-27: warning: nonterminal useless in grammar: call_stat [-Wother]
        | call_stat
                   ^^^^^^^^^
gram.y:151.35-44: warning: nonterminal useless in grammar: actualpars [-Wother]
        : CALL IDENTIFIER actualpars
                                   ^^^^^^^^^^
gram.y:155.22-34: warning: nonterminal useless in grammar: actualparlist [-Wother]
        | '('actualparlist')'
                      ^^^^^^^^^^^^^
gram.y:158.19-31: warning: nonterminal useless in grammar: actualparitem [-Wother]
        : actualparitem 
                   ^^^^^^^^^^^^^
gram.y:120.26-34: warning: nonterminal useless in grammar: condition [-Wother]
        : IF '(' condition')' brack_or_stat elsepart
                          ^^^^^^^^^
gram.y:166.28-38: warning: nonterminal useless in grammar: or_boolterm [-Wother]
        : boolterm or_boolterm 
                            ^^^^^^^^^^^
gram.y:166.19-26: warning: nonterminal useless in grammar: boolterm [-Wother]
        : boolterm or_boolterm 
                   ^^^^^^^^
gram.y:173.30-43: warning: nonterminal useless in grammar: and_boolfactor [-Wother]
        : boolfactor and_boolfactor
                              ^^^^^^^^^^^^^^
gram.y:173.19-28: warning: nonterminal useless in grammar: boolfactor [-Wother]
        : boolfactor and_boolfactor
                   ^^^^^^^^^^
gram.y:145.30-39: warning: nonterminal useless in grammar: expression [-Wother]
        : RETURN '(' expression ')'
                              ^^^^^^^^^^
gram.y:185.38-50: warning: nonterminal useless in grammar: add_oper_term [-Wother]
        : optional_sign term add_oper_term
                                      ^^^^^^^^^^^^^
gram.y:185.33-36: warning: nonterminal useless in grammar: term [-Wother]
        : optional_sign term add_oper_term
                                 ^^^^
gram.y:192.26-40: warning: nonterminal useless in grammar: mul_oper_factor [-Wother]
        : factor mul_oper_factor
                          ^^^^^^^^^^^^^^^
gram.y:192.19-24: warning: nonterminal useless in grammar: factor [-Wother]
        : factor mul_oper_factor
                   ^^^^^^
gram.y:201.30-35: warning: nonterminal useless in grammar: idtail [-Wother]
        | IDENTIFIER idtail
                              ^^^^^^
gram.y:182.30-44: warning: nonterminal useless in grammar: relational_oper [-Wother]
        | expression relational_oper expression
                              ^^^^^^^^^^^^^^^
gram.y:188.19-26: warning: nonterminal useless in grammar: add_oper [-Wother]
        : add_oper term
                   ^^^^^^^^
gram.y:195.19-26: warning: nonterminal useless in grammar: mul_oper [-Wother]
        : mul_oper factor mul_oper_factor
                   ^^^^^^^^
gram.y:185.19-31: warning: nonterminal useless in grammar: optional_sign [-Wother]
        : optional_sign term add_oper_term
                   ^^^^^^^^^^^^^
gram.y:43.19-42: warning: rule useless in grammar [-Wother]
        | PROGRAM IDENTIFIER block
                   ^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:47.19-59: warning: rule useless in grammar [-Wother]
        : "{" declarations subprograms sequence "}"
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:50.19-24: warning: rule useless in grammar [-Wother]
        : %empty
                   ^^^^^^
gram.y:51.19-44: warning: rule useless in grammar [-Wother]
        | DECLARE varlist ENDDECLARE 
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:54.19-24: warning: rule useless in grammar [-Wother]
        : %empty
                   ^^^^^^
gram.y:55.19-45: warning: rule useless in grammar [-Wother]
        | assignment_stat identifier2
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:58.19-37: warning: rule useless in grammar [-Wother]
 identifier2    : ',' assignment_stat
                   ^^^^^^^^^^^^^^^^^^^
gram.y:59.19-37: warning: rule useless in grammar [-Wother]
        | ',' assignment_stat 
                   ^^^^^^^^^^^^^^^^^^^
gram.y:63.19-22: warning: rule useless in grammar [-Wother]
        : func
                   ^^^^
gram.y:64.19-33: warning: rule useless in grammar [-Wother]
        | subprograms ','  
                   ^^^^^^^^^^^^^^^
gram.y:67.19-47: warning: rule useless in grammar [-Wother]
        : PROCEDURE IDENTIFIER funcbody
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:68.19-46: warning: rule useless in grammar [-Wother]
        | FUNCTION IDENTIFIER funcbody
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:71.19-34: warning: rule useless in grammar [-Wother]
        : formalpars block
                   ^^^^^^^^^^^^^^^^
gram.y:74.19-24: warning: rule useless in grammar [-Wother]
        : '('')'
                   ^^^^^^
gram.y:75.19-39: warning: rule useless in grammar [-Wother]
        | '(' formalparlist ')'
                   ^^^^^^^^^^^^^^^^^^^^^
gram.y:79.19-31: warning: rule useless in grammar [-Wother]
        : formalparitem
                   ^^^^^^^^^^^^^
gram.y:80.19-35: warning: rule useless in grammar [-Wother]
        | formalparlist ','
                   ^^^^^^^^^^^^^^^^^
gram.y:84.19-31: warning: rule useless in grammar [-Wother]
        : IN IDENTIFIER
                   ^^^^^^^^^^^^^
gram.y:85.19-34: warning: rule useless in grammar [-Wother]
        | INOUT IDENTIFIER
                   ^^^^^^^^^^^^^^^^
gram.y:88.19-46: warning: rule useless in grammar [-Wother]
        : statement statement_sequence
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:91.19-30: warning: rule useless in grammar [-Wother]
        : ";"statement
                   ^^^^^^^^^^^^
gram.y:92.19-50: warning: rule useless in grammar [-Wother]
        | ';' statement statement_sequence
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:95.19-34: warning: rule useless in grammar [-Wother]
        : '{' sequence '}'
                   ^^^^^^^^^^^^^^^^
gram.y:98.19-30: warning: rule useless in grammar [-Wother]
        : brackets_seq
                   ^^^^^^^^^^^^
gram.y:99.19-27: warning: rule useless in grammar [-Wother]
        | statement
                   ^^^^^^^^^
gram.y:104.19-24: warning: rule useless in grammar [-Wother]
        : %empty
                   ^^^^^^
gram.y:105.19-33: warning: rule useless in grammar [-Wother]
        | assignment_stat
                   ^^^^^^^^^^^^^^^
gram.y:106.19-25: warning: rule useless in grammar [-Wother]
        | if_stat
                   ^^^^^^^
gram.y:107.19-31: warning: rule useless in grammar [-Wother]
        | do_while_stat
                   ^^^^^^^^^^^^^
gram.y:108.19-28: warning: rule useless in grammar [-Wother]
        | while_stat
                   ^^^^^^^^^^
gram.y:109.19-29: warning: rule useless in grammar [-Wother]
        | select_stat
                   ^^^^^^^^^^^
gram.y:110.19-27: warning: rule useless in grammar [-Wother]
        | exit_stat
                   ^^^^^^^^^
gram.y:111.19-29: warning: rule useless in grammar [-Wother]
        | return_stat
                   ^^^^^^^^^^^
gram.y:112.19-28: warning: rule useless in grammar [-Wother]
        | print_stat
                   ^^^^^^^^^^
gram.y:113.19-27: warning: rule useless in grammar [-Wother]
        | call_stat
                   ^^^^^^^^^
gram.y:116.19-45: warning: rule useless in grammar [-Wother]
            : IDENTIFIER EXACT IDENTIFIER 
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:120.19-60: warning: rule useless in grammar [-Wother]
        : IF '(' condition')' brack_or_stat elsepart
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:123.19-24: warning: rule useless in grammar [-Wother]
        : %empty
                   ^^^^^^
gram.y:124.19-36: warning: rule useless in grammar [-Wother]
        | ELSE brack_or_stat
                   ^^^^^^^^^^^^^^^^^^
gram.y:127.19-54: warning: rule useless in grammar [-Wother]
        : WHILE '(' condition')' brack_or_stat
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:130.19-132.42: warning: rule useless in grammar [-Wother]
        : SELECT'(' IDENTIFIER')'
                   ^^^^^^^^^
gram.y:135.19-52: warning: rule useless in grammar [-Wother]
        : DECIMAL_CONSTANT ':' brack_or_stat
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:136.19-25: warning: rule useless in grammar [-Wother]
        | dec ','
                   ^^^^^^^
gram.y:139.19-58: warning: rule useless in grammar [-Wother]
        : DO brack_or_stat WHILE '(' condition ')'
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:142.19-22: warning: rule useless in grammar [-Wother]
        : EXIT
                   ^^^^
gram.y:145.19-43: warning: rule useless in grammar [-Wother]
        : RETURN '(' expression ')'
                   ^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:148.19-41: warning: rule useless in grammar [-Wother]
        : PRINT '('expression ')'
                   ^^^^^^^^^^^^^^^^^^^^^^^
gram.y:151.19-44: warning: rule useless in grammar [-Wother]
        : CALL IDENTIFIER actualpars
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:154.19-24: warning: rule useless in grammar [-Wother]
        : '('')'
                   ^^^^^^
gram.y:155.19-37: warning: rule useless in grammar [-Wother]
        | '('actualparlist')'
                   ^^^^^^^^^^^^^^^^^^^
gram.y:158.19-31: warning: rule useless in grammar [-Wother]
        : actualparitem 
                   ^^^^^^^^^^^^^
gram.y:159.19-35: warning: rule useless in grammar [-Wother]
        | actualparlist ','
                   ^^^^^^^^^^^^^^^^^
gram.y:162.19-31: warning: rule useless in grammar [-Wother]
        : IN expression
                   ^^^^^^^^^^^^^
gram.y:163.19-34: warning: rule useless in grammar [-Wother]
        | INOUT IDENTIFIER
                   ^^^^^^^^^^^^^^^^
gram.y:166.19-38: warning: rule useless in grammar [-Wother]
        : boolterm or_boolterm 
                   ^^^^^^^^^^^^^^^^^^^^
gram.y:169.19-29: warning: rule useless in grammar [-Wother]
        : OR boolterm
                   ^^^^^^^^^^^
gram.y:170.19-41: warning: rule useless in grammar [-Wother]
        | OR boolterm or_boolterm
                   ^^^^^^^^^^^^^^^^^^^^^^^
gram.y:173.19-43: warning: rule useless in grammar [-Wother]
        : boolfactor and_boolfactor
                   ^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:176.19-32: warning: rule useless in grammar [-Wother]
        : AND boolfactor
                   ^^^^^^^^^^^^^^
gram.y:177.19-47: warning: rule useless in grammar [-Wother]
        | AND boolfactor and_boolfactor
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:180.19-39: warning: rule useless in grammar [-Wother]
        : NOT '[' condition ']'
                   ^^^^^^^^^^^^^^^^^^^^^
gram.y:181.19-35: warning: rule useless in grammar [-Wother]
        | '[' condition ']'
                   ^^^^^^^^^^^^^^^^^
gram.y:182.19-55: warning: rule useless in grammar [-Wother]
        | expression relational_oper expression
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:185.19-50: warning: rule useless in grammar [-Wother]
        : optional_sign term add_oper_term
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:188.19-31: warning: rule useless in grammar [-Wother]
        : add_oper term
                   ^^^^^^^^^^^^^
gram.y:189.19-45: warning: rule useless in grammar [-Wother]
        | add_oper term add_oper_term
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:192.19-40: warning: rule useless in grammar [-Wother]
        : factor mul_oper_factor
                   ^^^^^^^^^^^^^^^^^^^^^^
gram.y:195.19-49: warning: rule useless in grammar [-Wother]
        : mul_oper factor mul_oper_factor
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gram.y:196.19-33: warning: rule useless in grammar [-Wother]
        | mul_oper factor 
                   ^^^^^^^^^^^^^^^
gram.y:199.19-34: warning: rule useless in grammar [-Wother]
        : DECIMAL_CONSTANT
                   ^^^^^^^^^^^^^^^^
gram.y:200.19-28: warning: rule useless in grammar [-Wother]
        | expression
                   ^^^^^^^^^^
gram.y:201.19-35: warning: rule useless in grammar [-Wother]
        | IDENTIFIER idtail
                   ^^^^^^^^^^^^^^^^^
gram.y:204.19-24: warning: rule useless in grammar [-Wother]
        : %empty
                   ^^^^^^
gram.y:205.19-28: warning: rule useless in grammar [-Wother]
        | actualpars
                   ^^^^^^^^^^
gram.y:208.19-21: warning: rule useless in grammar [-Wother]
        : '='
                   ^^^
gram.y:209.19-21: warning: rule useless in grammar [-Wother]
        | '<'
                   ^^^
gram.y:210.19-22: warning: rule useless in grammar [-Wother]
        | "<="
                   ^^^^
gram.y:211.19-22: warning: rule useless in grammar [-Wother]
        | "<>"
                   ^^^^
gram.y:212.19-22: warning: rule useless in grammar [-Wother]
        | ">="
                   ^^^^
gram.y:213.19-21: warning: rule useless in grammar [-Wother]
        | '>'
                   ^^^
gram.y:216.19-21: warning: rule useless in grammar [-Wother]
        : '+'
                   ^^^
gram.y:217.19-21: warning: rule useless in grammar [-Wother]
        | '-'
                   ^^^
gram.y:220.19-21: warning: rule useless in grammar [-Wother]
        : '*'
                   ^^^
gram.y:221.19-21: warning: rule useless in grammar [-Wother]
        | '/'
                   ^^^
gram.y:224.19-26: warning: rule useless in grammar [-Wother]
        : add_oper
                   ^^^^^^^^
gram.y:225.19-24: warning: rule useless in grammar [-Wother]
        | %empty
                   ^^^^^^

能帮我吗？

1 个解决方案

解决方案1
 2 已采纳  2017-05-01 03:35:53

问题是您生产的subprograms 。

但是，这对于您（以及可能偶然发现此问题和答案的任何同学）了解如何自行发现此类问题可能更有用。

StackOverflow鼓励您提供一个“最小，完整和可验证的示例”（MCVE） ，您应该花一点时间阅读该帮助页面。 要求不是因为我们喜欢让有疑问的人生活困难。 这是因为将问题减少到最低程度是调试的关键部分，并且您会发现，随着习惯的发展，发现问题的能力越来越强。 当然，由于它消除了所有不相关的噪音，它还可以帮助尝试解决问题的任何人。

但是MCVE不仅很小。 它也是完整的 ：也就是说，这实际上是一个问题的摘录。 仅仅说“我认为问题就在这里，所以我将仅演示这一部分”是不够的。 您必须通过创建最少的示例并证明确实会产生相同的问题，来证明问题出在哪里。

实际上，如果您牢记这一策略，则将以不同的方式编写程序。 您无需编写一千行代码，然后将其全部输入到编译器中，而是将编写一些独立的部分并检查每个部分，然后再将它们组合为一个整体。 这样，当您无法解决特定问题时，就已经拥有MCVE。

语法令人讨厌地相互联系，并且不像人们所希望的程序那样模块化。 一个生产中的少量更改会产生移位减少冲突，涉及另一个看似完全无关的生产。 但是，您通常可以将语法分成或多或少的独立部分。 例如，您可以仅从定义表达式的语法部分开始： 

%token PROGRAM
%token IDENTIFIER 
%token DECIMAL_CONSTANT
%token DECLARE ENDDECLARE

%token AND CALL DEFAULT
%token FUNCTION PROCEDURE IN INOUT
%token IF ELSE DO WHILE FOR OR NOT SELECT
%token RETURN EXIT  PRINT 
%token EQ_LT EQ_GT NE EXACT 
%left '+' '-'
%right '*' '/'

%%

expression
        : optional_sign term add_oper_term
        ;
add_oper_term
        : add_oper term
        | add_oper term add_oper_term
        ;
term        
        : factor mul_oper_factor
        ;
mul_oper_factor
        : mul_oper factor 
        | mul_oper factor mul_oper_factor
        ;
factor
        : DECIMAL_CONSTANT
        | expression
        | IDENTIFIER idtail
        ;
idtail
        : %empty
        | actualpars
        ;
add_oper
        : '+'
        | '-'
        ;
mul_oper
        : '*'
        | '/'
        ;
optional_sign
        : add_oper
        | %empty
        ;

actualpars
        : '('')'
        | '('actualparlist')'
        ;
actualparlist
        : actualparitem 
        | actualparlist ','
        ;
actualparitem
        : IN expression
        | INOUT IDENTIFIER
        ;

这就产生了许多移位/减少冲突，您将必须解决这些冲突，但是却没有任何无用的产生。 （请注意，我将所有%token声明留在了适当的位置，因此上述内容并不是很简单。Bison不在乎是否声明了您从未使用过的终端，因此将声明保留在以后的步骤中会更容易）

向上移动一步，我们可以添加condition ，其中涉及添加： 

condition
        : boolterm or_boolterm 
        ;
or_boolterm 
        : OR boolterm
        | OR boolterm or_boolterm
        ;
boolterm    
        : boolfactor and_boolfactor
        ;
and_boolfactor
        : AND boolfactor
        | AND boolfactor and_boolfactor
        ;
boolfactor  
        : NOT '[' condition ']'
        | '[' condition ']'
        | expression relational_oper expression
        ;
relational_oper
        : '='
        | '<'
        | "<="
        | "<>"
        | ">="
        | '>'
        ;

现在有更多的移位/减少冲突，但仍然没有无用的非终结符。

下一步将添加语句产生。 在这里，我们会发现许多语句使用了非终结brack_or_stat ，它的定义如下： 

brack_or_stat   
    : brackets_seq
    | statement
    ;

为了避免再次处理brackets_seq ，我们可以将其临时添加到终端列表中。 （这是隐藏语法细节的标准技术。）稍后，当我们包含brackets_seq ，我们可以删除%token声明。）因此，我们现在添加 

%token brackets_seq

before the `%%`, and the statement productions after it:

brack_or_stat   
        : brackets_seq
        | statement
        ;

statement   
        : %empty
        | assignment_stat
        | if_stat
        | do_while_stat
        | while_stat
        | select_stat
        | exit_stat
        | return_stat
        | print_stat
        | call_stat
        ;
assignment_stat
            : IDENTIFIER EXACT IDENTIFIER 
            ;

if_stat
        : IF '(' condition')' brack_or_stat elsepart
        ;
elsepart
        : %empty
        | ELSE brack_or_stat
        ;
while_stat
        : WHILE '(' condition')' brack_or_stat
        ;
select_stat
        : SELECT'(' IDENTIFIER')'
          dec 
          DEFAULT":" brack_or_stat
        ;
dec
        : DECIMAL_CONSTANT ':' brack_or_stat
        | dec ','
        ;
do_while_stat
        : DO brack_or_stat WHILE '(' condition ')'
        ;
exit_stat   
        : EXIT
        ;
return_stat
        : RETURN '(' expression ')'
        ;
print_stat
        : PRINT '('expression ')'
        ;
call_stat
        : CALL IDENTIFIER actualpars
        ;

现在有更多的移位/减少冲突，但是没有无用的非终结符。

因此，现在我们已经研究了一半以上的语法，并且有把握地确信问题不在我们所研究的部分中。 （尽管还有很多其他问题需要解决。）

因此，如果我们假设该statement还可以，那么将其设为终端，然后从测试中删除该引用以及它引用的所有非终端。 这给我们留下了一个简单得多的程序：（当我复制并粘贴此摘录并通过野牛运行时，它通知我未定义assignment_stat 。因此，我将其添加到%token声明中。） 

%token PROGRAM
%token IDENTIFIER 
%token DECIMAL_CONSTANT
%token DECLARE ENDDECLARE

%token AND CALL DEFAULT
%token FUNCTION PROCEDURE IN INOUT
%token IF ELSE DO WHILE FOR OR NOT SELECT
%token RETURN EXIT  PRINT 
%token EQ_LT EQ_GT NE EXACT 

%token statement assignment_stat

%start program
%%

program
        : %empty
        | PROGRAM IDENTIFIER block
        ;

block
        : "{" declarations subprograms sequence "}"
        ;
declarations
        : %empty
        | DECLARE varlist ENDDECLARE 
        ;
varlist
        : %empty
        | assignment_stat identifier2
        ;

identifier2 : ',' assignment_stat
        | ',' assignment_stat 

        ;
subprograms 
        : func
        | subprograms ','  
        ;
func        
        : PROCEDURE IDENTIFIER funcbody
        | FUNCTION IDENTIFIER funcbody
        ;
funcbody    
        : formalpars block
        ;
formalpars  
        : '('')'
        | '(' formalparlist ')'

        ;
formalparlist   
        : formalparitem
        | formalparlist ','

        ;
formalparitem   
        : IN IDENTIFIER
        | INOUT IDENTIFIER
        ;       
sequence
        : statement statement_sequence
        ;
statement_sequence
        : ";"statement
        | ';' statement statement_sequence
        ;
brackets_seq    
        : '{' sequence '}'
        ;
brack_or_stat   
        : brackets_seq
        | statement
        ;

现在，我们有一个充满“无用的非终端”警告的屏幕。

但是，语法现在要简单得多。 （如果看起来不够简单，则可以继续进行简化操作。例如，一种可能性是分离声明。）确实，问题可能很明显： 

subprograms 
        : func
        | subprograms ','  
func    : PROCEDURE IDENTIFIER funcbody
        | FUNCTION IDENTIFIER funcbody
funcbody: formalpars block
block: "{" declarations subprograms sequence "}"

注意这里发生了什么： subprograms有两个生成。 第二个显然是递归的。 但是，只需花很少的工作就可以看到另一个也是递归的： func有两个产品，但是都使用funcbody 。 funcbody的唯一产品使用block ，而block使用subprograms 。

因此， subprograms没有非递归产生，因此它永远不会脱离递归。 而且func ， funcbody或block都不能。

但是，如果block无法生成任何语句，则program的第二个产生式（ PROGRAM IDENTIFIER block ）也无法生成任何语句，从而使program: %empty成为起始符号的唯一有效产生。 这使空字符串成为唯一有效的program ，因此，野牛将所有其他非终端标记为无用。

现在，让我们回到subprograms （正如我刚开始所说的那样，这是当前的问题）。 定义是： 

subprograms 
        : func
        | subprograms ','

现在，这是一个非常奇怪的定义（我认为，整个语法中都会重复出现这种奇怪的现象）。 这是在说什么 该subprograms是一个func或一个subprograms后跟一个逗号。 因此它将产生： 

func
func ,
func , ,
func , , ,
func , , , ,

等等。 单个func ，后跟零个或多个逗号。 它必须具有一个func，而不能具有更多。

它不能为空的事实是无限递归的原因。 block必须包含一个subprograms ，并且一个subprograms必须包含一个func 。 但func必须包含一个block 。 从那个旋转木马也无法逃脱。

因此，这里存在两个问题： subprograms不能为空，也不能包含多个func 。 您可能正在寻找的是： 

function_list: func
             | function_list ',' func
subprograms  : %empty
             | function_list

请注意， function_list使用左递归。 那是故意的。 野牛产生LR解析器，它喜欢左递归。

如果您以前学习过LL解析器，却以某种方式使它陷入困境，那么左递归就是邪恶的， 那就不要上那课了 。 LL解析器无法处理左递归，但是LR解析器可以处理左递归和右递归，并且可以更好地处理左递归（因为它不会耗尽解析器堆栈）。

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