Bison: Conflicts: 1 shift/reduce error - parsing

Question

I'm trying to build a parser with bison and have narrowed all my errors down to one difficult one.
Here's the debug output of bison with the state where the error lies:
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)
Here are the translation rules in the parser.y source
%%
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
;
%%
When I remove the if_statement rule there are no errors, so I've narrowed it down considerably, but still can't solve the error.
Thanks for any help.

Answer 1

Consider this statement: if condition then s2 else s3; s4
There are two interpretations:
if condition then
s1;
else
s2;
s3;
The other one is:
if condition then
s1;
else
s2;
s3;
In the first one, the statment list is composed of an if statement and s3. While the other statement is composed of only one if statement. That's where the ambiguity comes from. Bison will prefer shift to reduce when a shift-reduce conflict exist, so in the above case, the parser will choose to shift s3.
Since you have an ENDIF in your if-then statement, consider to introduce an ENDIF in your if-then-else statement, then the problem is solved.

Answer 2

I think you are missing ENDIF in the IF-THEN-ELSE-ENDIF rule.