Bison Shift/Reduce conficts

Question

//parserr.y 
//tokens


---------
/* precedences */
%left T_LPAREN T_RPAREN 
%left T_MULOP T_DIVOP
%left T_ADDOP
%left T_ANDOP
%left T_OROP
%left T_NOTOP
%right T_POWEROP
%right T_ASSIGN


%nonassoc T_RELOP
%nonassoc T_PAIROP
%start program 

** syntax rules - same file **

program:  body T_END subprograms
;
body:  declarations statements
;
declarations: declarations couplespec type vars
| declarations T_DATA vals
| %empty { }
;
couplespec: T_COUPLE
| %empty { }
;
type: T_INTEGER
| T_REAL
| T_LOGICAL
| T_CHARACTER
;
vars: vars T_COMMA undef_variable
| undef_variable
;
undef_variable: T_ID T_LPAREN dims T_RPAREN         
| T_ID                                
;
dims: dims T_COMMA dim
| dim
;
dim: T_ICONST
| T_ID                                
;
vals: vals T_COMMA T_ID value_list      
| T_ID value_list                   
;
value_list: T_DIVOP values T_DIVOP
;
values: values T_COMMA value
| value
;
value: repeat T_MULOP T_ADDOP simp_constant
| repeat T_MULOP constant
| repeat T_MULOP T_STRING
| T_ADDOP simp_constant
| constant
| T_STRING
;
repeat: T_ICONST
| %empty { }
;
simp_constant: T_ICONST
| T_RCONST
| T_LCONST
| T_CCONST
;
constant: simp_constant
| coup_constant
;

coup_constant: T_LPAREN simp_constant T_COLON simp_constant T_RPAREN
| T_LPAREN simp_constant T_COLON T_ADDOP simp_constant T_RPAREN
| T_LPAREN T_ADDOP simp_constant T_COLON simp_constant T_RPAREN
| T_LPAREN T_ADDOP simp_constant T_COLON T_ADDOP simp_constant T_RPAREN
;
statements: statements labeled_statement
| labeled_statement
;
labeled_statement: label statement
| statement
;
label: T_ICONST
;
statement: simple_statement
| compound_statement
;
simple_statement: assignment
| goto_statement
| if_statement
| subroutine_call
| io_statement
| T_CONTINUE
| T_RETURN
| T_STOP
;
assignment: variable T_ASSIGN expression
| variable T_ASSIGN T_STRING
;
variable: variable T_LPAREN expressions T_RPAREN
| T_ID                                      
;
expressions: expressions T_COMMA expression
| expression
;
expression: expression T_OROP expression
| expression T_ANDOP expression
| expression T_RELOP expression
| expression T_ADDOP expression
| expression T_MULOP expression
| expression T_DIVOP expression
| expression T_POWEROP expression
| T_NOTOP expression
| T_ADDOP expression
| T_PAIROP expression
| variable
| constant
| T_LPAREN expression T_COLON expression T_RPAREN
| T_LPAREN expression T_RPAREN
;
goto_statement: T_GOTO label
| T_GOTO T_ID T_COMMA T_LPAREN labels T_RPAREN            
;
labels: labels T_COMMA label
| label
;
if_statement: T_IF T_LPAREN expression T_RPAREN label T_COMMA label T_COMMA label
| T_IF T_LPAREN expression T_RPAREN simple_statement
| T_IF error expression T_RPAREN simple_statement           
| T_IF T_LPAREN expression error simple_statement            
;
subroutine_call: T_CALL variable
;
io_statement: T_READ read_list
| T_WRITE write_list
;
read_list: read_list T_COMMA read_item
| read_item
;
read_item: variable
| T_LPAREN read_list T_COMMA T_ID T_ASSIGN iter_space T_RPAREN         
;
iter_space: expression T_COMMA expression step
;
step: T_COMMA expression
| %empty { }
;
write_list: write_list T_COMMA write_item
| write_item
;
write_item: expression
| T_LPAREN write_list T_COMMA T_ID T_ASSIGN iter_space T_RPAREN       
| T_STRING
;
compound_statement: branch_statement
| loop_statement
;
branch_statement: T_IF T_LPAREN expression T_RPAREN T_THEN body tail
;
tail: T_ELSE body T_ENDIF
| T_ENDIF
;
loop_statement: T_DO T_ID T_ASSIGN iter_space body T_ENDDO                      
;
subprograms: subprograms subprogram
| %empty { }
;
subprogram: header body T_END
;
header: type T_FUNCTION T_ID T_LPAREN formal_parameters T_RPAREN                
| T_SUBROUTINE T_ID T_LPAREN formal_parameters T_RPAREN                   
| T_SUBROUTINE T_ID                                                       
;
formal_parameters: type vars T_COMMA formal_parameters
| type vars
;

(文件：parserr.output(

状态87冲突：1个班次/减少

状态124冲突：1个移位/减少

(状态87(：

State 87
37 constant: simp_constant .
39 coup_constant: "(" simp_constant . ":" simp_constant ")"
40              | "(" simp_constant . ":" "+ -" simp_constant ")"

":"  shift, and go to state 125
":"       [reduce using rule 37 (constant)]
$default  reduce using rule 37 (constant)

(状态124(：

State 124
37 constant: simp_constant .
41 coup_constant: "(" "+ -" simp_constant . ":" simp_constant ")"
42              | "(" "+ -" simp_constant . ":" "+ -" simp_constant ")"
":"  shift, and go to state 159
":"       [reduce using rule 37 (constant)]
$default  reduce using rule 37 (constant)

我得到了2次轮班/减少冲突，我最终尝试使用%prec XXX，但没有成功。我还尝试定义冒号"："使用左优先，它也不起作用。

• 已更改：-来自%nonasso LPAREN RPAREN=>TO%左LPAREN RPAREN

Answer 1

您的语法在expression:的规则中有歧义

expression: T_LPAREN expression T_COLON expression T_RPAREN
expression: constant

带有

constant: coup_constant
coup_constant: T_LPAREN simp_constant T_COLON simp_constant T_RPAREN

(1,2)可以通过其中任何一个生成，从而导致冲突。

最简单的解决方案是用expression: simp_constant替换expression: constant，迫使解析器接受更通用的选项。

在可能的情况下尝试使用常量解释将导致无法正确解析(2:a)，这是一个有效的expression，因为解析器需要在a成为前瞻之前做出决定。出于同样的原因，您无法通过优先级声明轻松解决此冲突。

这可能符合也可能不符合您的语义需求，但您可以在必要时转换语义操作中的常量。