Useless rule in Bison - c++

Question

For some reason bison is rejecting a specific rule, the notequal_expression, beware that Im just starting to learn the whole concept so my line of thought is not so mature, the input file: ( The Error is: "string.y contains 1 useless nonterminal and 1 useless rule." )
/* Parser for StringC */
%{
/* ------------------------------------------------------------------
Initial code (copied verbatim to the output file)
------------------------------------------------------------------ */
// Includes
#include <malloc.h> // _alloca is used by the parser
#include <string.h> // strcpy
#include "lex.h" // the lexer
// Some yacc (bison) defines
#define YYDEBUG 1 // Generate debug code; needed for YYERROR_VERBOSE
#define YYERROR_VERBOSE // Give a more specific parse error message
// Error-reporting function must be defined by the caller
void Error (char *format, ...);
// Forward references
void yyerror (char *msg);
%}
/* ------------------------------------------------------------------
Yacc declarations
------------------------------------------------------------------ */
/* The structure for passing value between lexer and parser */
%union {
char *str;
}
%token ERROR_TOKEN IF ELSE PRINT INPUT ASSIGN EQUAL NOTEQUAL
%token CONCAT END_STMT OPEN_PAR CLOSE_PAR
%token BEGIN_CS END_CS
%token <str> ID STRING BOOLEAN
/*%type <type> type simple_type cast*/
%expect 1 /* shift/reduce conflict: dangling ELSE */
/* declaration */
%%
/* ------------------------------------------------------------------
Yacc grammar rules
------------------------------------------------------------------ */
program
: statement_list
;
statement_list
: statement_list statement
| /* empty */
;
statement
: END_STMT {puts ("Empty statement");}
| expression END_STMT {puts ("Expression statement");}
| PRINT expression END_STMT {puts ("Print statement");}
| INPUT identifier END_STMT {puts ("Input statement");}
| if_statement {puts ("If statement");}
| compound_statement {puts ("Compound statement");}
| error END_STMT {puts ("Error statement");}
| notequal_expression {puts ("Not equal statement");}
;
/* NOTE: This rule causes an unresolvable shift/reduce conflict;
That's why %expect 1 was added (see above) */
if_statement
: IF OPEN_PAR expression CLOSE_PAR statement optional_else_statement
;
optional_else_statement
: ELSE statement
| /* empty */
;
compound_statement
: BEGIN_CS statement_list END_CS
;
expression
: equal_expression
| OPEN_PAR expression CLOSE_PAR
;
equal_expression
: expression EQUAL assign_expression
| assign_expression
;
notequal_expression
: expression NOTEQUAL assign_expression
| NOTEQUAL assign_expression
;
assign_expression
: identifier ASSIGN assign_expression
| concat_expression
;
concat_expression
: concat_expression CONCAT simple_expression
| simple_expression
;
simple_expression
: identifier
| string
;
identifier
: ID {}
;
string
: STRING {}
;
bool
: BOOLEAN {}
;
%%
/* ------------------------------------------------------------------
Additional code (again copied verbatim to the output file)
------------------------------------------------------------------ */
The lexer:
/* Lexical analyzer for StringC */
%{
/* ------------------------------------------------------------------
Initial code (copied verbatim to the output file)
------------------------------------------------------------------ */
// Includes
#include <string.h> // strcpy, strncpy
#include <io.h> // isatty
#ifdef MSVC
#define isatty _isatty // for some reason isatty is called _isatty in VC..
#endif
#define _LEX_CPP_ // make sure our variables get created
#include "lex.h"
#include "lexsymb.h"
extern "C" int yywrap (); // the yywrap function is declared by the caller
// Forward references
void Identifier ();
void StringConstant ();
void BoolConstant ();
void EatComment ();
//// End of inititial code
%}
/* ------------------------------------------------------------------
Some macros (standard regular expressions)
------------------------------------------------------------------ */
LETTER [a-zA-Z_]
DIGIT [0-9]
IDENT {LETTER}({LETTER}|{DIGIT})*
STR \"[^\"]*\"
BOOL \(false|true)\
WSPACE [ \t]+
/* ------------------------------------------------------------------
The lexer rules
------------------------------------------------------------------ */
%%
"if" {return IF;}
"else" {return ELSE;}
"print" {return PRINT;}
"input" {return INPUT;}
"=" {return ASSIGN;}
"==" {return EQUAL;}
"!=" {return NOTEQUAL;} /* Not equal to */
"+" {return CONCAT;}
";" {return END_STMT;}
"(" {return OPEN_PAR;}
")" {return CLOSE_PAR;}
"{" {return BEGIN_CS;}
"}" {return END_CS;}
{BOOL} {BoolConstant (); return BOOLEAN;}
{STR} {StringConstant (); return STRING;}
{IDENT} {Identifier (); return ID;}
"//" {EatComment();} /* comment: skip */
\n {lineno++;} /* newline: count lines */
{WSPACE} {} /* whitespace: (do nothing) */
. {return ERROR_TOKEN;} /* other char: error, illegal token */
%%
/* ------------------------------------------------------------------
Additional code (again copied verbatim to the output file)
------------------------------------------------------------------ */
// The comment-skipping function: skip to end-of-line
void EatComment() {
char c;
while ((c = yyinput()) != '\n' && c != 0);
lineno++;
}
// Pass the id name
void Identifier () {
yylval.str = new char[strlen(yytext)+1];
strcpy (yylval.str, yytext);
}
// Pass the string constant
void StringConstant() {
int l = strlen(yytext)-2;
yylval.str = new char[l+1];
strncpy (yylval.str, &yytext[1], l); yylval.str[l] = 0;
}
void BoolConstant() {
int l = strlen(yytext)-2;
yylval.str = new char[l+1];
strncpy(yylval.str, &yytext[1], l); yylval.str[l] = 0;
}

Answer 1

Are you sure that it's notequal_expression that is causing the issue? The nonterminal and rule that are not used, as I read it, are
bool
: BOOLEAN {}
;
Perhaps instead of
simple_expression
: identifier
| string
;
you intended to code
simple_expression
: identifier
| string
| bool
;

Answer 2

There are two problems with the grammar. The first is the shift/reduce conflict you've already seen (and addressed with %expect 1. I prefer to address it in the grammar instead and use %expect 0 instead. You can do that by removing ELSE from the %token list and adding a line
%right THEN ELSE
To declare right associativity. Your language doesn't actually have a THEN keyword but that's fine. You can then remove completely the rule for optional_else_statement and reword the rule for if_statement as follows:
if_statement
: IF OPEN_PAR expression CLOSE_PAR statement %prec THEN
| IF OPEN_PAR expression CLOSE_PAR statement ELSE statement
;
There are those who prefer to resolve it this way, and others who advocate the %expect 1 approach. I prefer this way, but now that you have both methods, you can certainly choose for yourself.
For the other problem, the useless rule is definitely this one:
bool
: BOOLEAN {}
;
because the non-terminal bool is not used anywhere else in the grammar. This accounts for both "1 useless nonterminal and 1 useless rule" as reported by bison. To be able to identify this kind of thing for yourself, you can use
bison --report=solved -v string.y
This will create a string.output file which will contain a large but readable report including any resolved shift-reduce errors (such as your IF-ELSE construction) and also a complete set of states created by bison. It's very useful when attempting to troubleshoot grammar problems.