Useless rule due to conflicts in YACC

Question

The error message:

bison -vdy tjc.y
conflicts: 2 shift/reduce
tjc.y:72.26-29: warning: rule useless in parser due to conflicts: return_type: type

EDIT 2: Further reduced the grammar to only the following rules. Same error due to same conflict. If I remove field_decl in the rules then the conflict goes away.

member_decl:      field_decl |  method_decl;

field_decl: STATIC type IDENT EQ SEMI;

method_decl:      STATIC return_type IDENT LPAR RPAR;

type:             INT | FLOAT;

return_type:      type | VOID;

Here's the section with the shift/reduce conflict in y.output:

state 18

7 field_decl: STATIC type . IDENT EQ SEMI
14 return_type: type .

IDENT  shift, and go to state 23

IDENT  [reduce using rule 14 (return_type)]

Please help me see what's going wrong here.

Is it possible that you're not mentioning that a type can be an ident as well? Is this really your (unedited) grammar? — Perry
I added a rule for type->IDENT and it produced more conflicts. And yes it is a part of the unedited grammar. This is a very simplified parser. — Leif
Could you manage to reduce this to the smallest grammar that displays the same error message? — Perry
Try reducing it further. Does the error remain if you remove the last three forms of method_decl? can you remove the field_decl rule and its referents etc? We're trying to get to the smallest grammar that exhibits the error. — Perry

Jonathan Leffler Jonathan Leffler · Accepted Answer · 2012-02-28T01:58:24

I think Perry's analysis of why you have problems is correct.

I was able to get similar warnings with this complete grammar (identical to yours, but with %token rules in place so I can compile it):

%token STATIC IDENT FLOAT INT VOID LPAR RPAR EQ SEMI

%%
member_decl:    field_decl |  method_decl;
field_decl:     STATIC type IDENT EQ SEMI;
method_decl:    STATIC return_type IDENT LPAR RPAR;
type:           INT | FLOAT;
return_type:    type | VOID;
%%

Given the analysis, the way I would probably fix it is this:

%token STATIC IDENT FLOAT INT VOID LPAR RPAR EQ SEMI

%%
member_decl:    field_decl |  method_decl;
field_decl:     STATIC type IDENT EQ SEMI;
method_decl:    STATIC type IDENT LPAR RPAR;
type:           INT | FLOAT | VOID;
%%

This compiles without any error in Bison. You just need to add a semantic check in the processing for field_decl that ensures that the relevant type ($2) is not VOID.

Useless rule due to conflicts in YACC

2 Answers