if-else statement in bison

Question

I'm trying to do the if-then statement in bison. The problem is that im working on a solution that everyone says that is working, but it doesnt :( my code is:

statement : matchedstmt
| unmatchedstmt
; matchedstmt : if '(' expression ')' matchedstmt else matchedstmt

| otherstmt                 
;

unmatchedstmt : if '(' expression ')' statement

| if '('expression ')' matchedstmt else unmatchedstmt   
;

otherstmt : expressionstmt

| compoundstmt  

| iterationstmt     

| returnstmt    
;

...

where "if" & "else" are %token

!! in terminal it says that i have one shift/reduce.

I also tried %nonassoc and %left

what can i do??

Answer 1

the best way for solve this problem using %nonassoc.

%nonassoc THEN

%nonassoc ELSE

%%


statement:              TIF TLPAREN expression TRPAREN TTHEN statement %prec THEN

                        | TIF TLPAREN expression TRPAREN TTHEN statement TELSE statement


%%

Answer 2

The following can be processed by bison with no conflicts of any kind:

statement : matchedstmt     
    | unmatchedstmt                 
    ;
matchedstmt : if '(' expression ')' matchedstmt else matchedstmt 
    | otherstmt                 
    ;
unmatchedstmt : if '(' expression ')' statement 
    | if '('expression ')' matchedstmt else unmatchedstmt   
    ;
otherstmt :  expressionstmt 
    | compoundstmt  
    | iterationstmt     
    | returnstmt    
    ;

That's unsurprising since you are using a standard mechanism for disambiguating the if ... else statement.

Presumably, the shift-reduce conflict is somewhere else in your grammar, possibly involving an interaction with this fragment. I suggest you add more statement types one at a time until you find the rule which causes the conflict. Unfortunately, LR grammars do not compose well: it is quite possible for two perfectly conflict-free fragments to produce a conflict when they are combined in a grammar.