YACC 7 rules never reduced 15 reduce/reduce conflicts

Question

I am trying to make a basic compiler parse and interpret a simple made up programming language. I have my context free grammar created, and have tried to implement it in YACC, but I keep getting '6 fules never reduced'. Here are my productions in the yacc file

%token BEGIN END INT INT_CONST STRING STRING_LIT TERM ID PRINT

%%


start : program
      ;

program: ID 'is' compound_statement
                    ; 

compound_statement: BEGIN statement_loop END; {;}
                    ;

statement_loop: statement statement_loop
                | statement
                ;

statement:      ID '=' expr ';'
                | ID'=' STRING_LIT ';'
                | PRINT value_loop ';'
                ;

value_loop:     value '.' value_loop
                | value
                ;

value:          STRING_LIT
                | expr
                ;

expr:           term_loop
                | '-' term_loop
                ;

term_loop:      term
                | term '+' term_loop
                | term '-' term_loop
                ;

term:           factor_loop
                ;

factor_loop:    factor_loop
                | factor '*' factor_loop
                | factor '/' factor_loop
                ;

declaration:    INT id_loop
                | STRING id_loop
                ;

id_loop:        ID
                | id_loop ','
                ;

factor:         INT_CONST
                | ID
                '(' expr ')'
                ;

Answer 1

factor_loop has no non-recursive production, so it cannot ever produce anything. You probably meant the first production to be factor_loop: factor.

Since factor_loop can't produce anything, all of the productions which rely on factor_loop are also useless.

There are a number of other problems with that grammar, so other error messages are likely. I don't see anything which would cause reduce-reduce conflicts, which makes me wonder whether what you provided was the complete literal grammar file.

Some of the issues:

1. In program, 'is' is not a valid single-character token.

2. There is a stray semicolon after the END in compound_statement.

3. declaration is not used anywhere.

4. id_loop produces an ID followed by any number of commas. So a,,,, is a valid id_loop but not a, b, c.

5. I'm pretty sure you are missing a | at the beginning if the third line of factor.

6. There is way too much right recursion. In some cases, this only produces excess parser stack usage, but it is particularly serious for arithmetic operators because it makes right-associative and they aren't.

   In particular, it parses 1 - 2 - 3 as 1 - (2 - 3) instead of the correct (1 - 2) - 3

7. Unary minus has far too high a precedence. You will end up parsing -2 + 6 as though it had been written -(2 + 6), so the result will be -8 instead of 4.