Question

Hello I have just started doing some parsing in bison/yacc. Now my first program already fails. What went wrong? I am using an example from:
original source of tutorial

%{
    #include <stdio.h>
    int yylex(void);
    void yyerror(char *);
%}


%token INTEGER

%%

program:
        program expr '\n'         { printf("%d\n", $2); }
        | 
        ;

expr:
        INTEGER                   { $$ = $1; }
        | expr '+' expr           { $$ = $1 + $3; }
        | expr '-' expr           { $$ = $1 - $3; }
        ;

%%

void yyerror(char *s) {
    fprintf(stderr, "%s\n", s);
}

int main(void) {
    yyparse();
    return 0;
}

Using version 2.4.1 of bison I get this error:

conflicts: 4 shift/reduce

Answer 1

Try that:

expr:
    INTEGER                   { $$ = $1; }
    | expr '+' INTEGER           { $$ = $1 + $3; }
    | expr '-' INTEGER           { $$ = $1 - $3; }

bison/yacc don’t like right recursion. If the input is 1+2+3, when the parser reaches 1+2+ it can’t decide either to reduce 1+2 to expr or to shift another token in order to reduce 2+3 to expr. By specifying INTEGER on the right, it can decide to reduce as soon as it sees 1+2 leaving only expr + 3 then reducing again.

You can also solve the issue by specifying that + and - are left associative, thus giving a higher priority to reducing 1+2 over shifting new tokens. Add that line in your preamble:

%left '+' '-'

Hope it helps.