shift-reduce-conflict

I have the following yacc/bison/happy grammar: %token if TokenIf then TokenThen else TokenElse true TokenTrue false TokenFalse %left APP %right IF %% Hungry : NoHungry | Hungry NoHungry %prec APP | if Hungry then Hungry else Hungry %prec IF NoHungry : true | false bison -v tells me there are two conflicts in the following situation: State 12 2 Hungry: Hungry . NoHungry 3 | if Hungry then Hungry else Hungry . true shift, and go to state 2 false shift, and go to state 3 true [reduce using rule 3 (Hungry)] false [reduce using rule 3 (Hungry)] $default reduce using rule 3 (Hungry) NoHungry go to

I have written a yacc file according to Tiger Book(appendix A, Tiger manual). But there are still some shift/reduce conflicts. I do not know how to resolve these conflicts. % yacc --version bison (GNU Bison) 3.0.2 You can use this cmd to reproduce the problem: % yacc -dvt tiger.y tiger.y: warning: 37 shift/reduce conflicts [-Wconflicts-sr] % cat tiger.y : %{ #include <stdio.h> //#include "util.h" //#include "errormsg.h" int yylex(void); /* function prototype */ void yyerror(char *s) { EM_error(EM_tokPos, "%s", s); } %} %union { int pos; int ival; string sval; } %token <sval> ID STRING %token

A piece of code of my gramamar its driveing me crazy. I have to write a grammar that allow write functions with multiple inputs e.g. function begin a: <statments> b: <statements> end The problem with that its that is statements that are assignments like this ID = Expresion. in the following quote you can see the output produced by yacc. 0 $accept : InstanciasFuncion $end 1 InstanciasFuncion : InstanciasFuncion InstanciaFuncion 2 | InstanciaFuncion 3 InstanciaFuncion : PuntoEntrada Sentencias 4 PuntoEntrada : ID ':' 5 Sentencias : Sentencias Sentencia 6 | Sentencia 7 Sentencia : ID '=' ID State

The grammar is as follows: 1. program -> declaration-list 2. declaration-list -> declaration-list declaration | declaration 3. declaration -> var-declaration | fun-declaration 4. var-declaration -> type-specifier ID ; | type-specifier ID [ NUM ] ; 5. type-specifier -> int | void 6. fun-declaration -> type-specifier ID ( params ) compound-stmt 7. params -> param-list | void 8. param-list -> param-list , param | param 9. param -> type-specifier ID | type-specifier ID [ ] 10. compound-stmt -> { local-declarations statement-list } 11. local-declarations -> local-declarations var-declarations |

How do I remove shift-reduce conflict for bison for the given grammar? selection-stmt -> if ( expression ) statement | if ( expression ) statement else statement A solution giving the modified grammar would be highly appreciated. There is a much simpler solution. If you know how LR parsers work, then you know that the conflict happens here: if ( expression ) statement * else statement where the star marks the current position of the cursor. The question the parser must answer is "should I shift, or should I reduce". Usually, you want to bind the else to the closest if , which means you want to