Question

I’m having a problem understanding the shift/reduce confict for a grammar that I know has no ambiguity. The case is one of the if else type but it’s not the ‘dangling else’ problem since I have mandatory END clauses delimiting code blocks.

Here is the grammar for gppg (Its a Bison like compiler compiler … and that was not an echo):

%output=program.cs  %start program  %token FOR %token END %token THINGS %token WHILE %token SET %token IF %token ELSEIF %token ELSE %%  program : statements         ;  statements : /*empty */            | statements stmt            ;  stmt : flow      | THINGS      ;  flow : '#' IF '(' ')' statements else      ;  else : '#' END      | '#' ELSE statements '#' END      | elseifs      ;  elseifs : elseifs '#' ELSEIF statements else         | '#' ELSEIF statements else         ; 

Here is the conflict output:

// Parser Conflict Information for grammar file 'program.y'  Shift/Reduce conflict on symbol ''#'', parser will shift  Reduce 10: else -> elseifs  Shift ''#'':   State-22 -> State-23   Items for From-state State 22     10 else: elseifs .     -lookahead: '#', THINGS, EOF     11 elseifs: elseifs . '#' ELSEIF statements else    Items for Next-state State 23     11 elseifs: elseifs '#' . ELSEIF statements else   // End conflict information for parser 

I already switched arround everything, and I do know how to resolve it, but that solution involves giving up the left recursion on ‘elseif’ for a right recursion.

Ive been through all the scarse documentation I have found on the internet regarding this issue (I post some links at the end) and still have not found an elegant solution. I know about ANTLR and I don’t want to consider it right now. Please limit your solution to Yacc/Bison parsers.

I would appreciate elegant solutions, I managed to do It by eleminating the /* empty */ rules and duplication everything that needed an empty list but in the larger grammar Im working on It just ends up like ‘sparghetti grammar syndrome’.

Here are some links:

http://nitsan.org/~maratb/cs164/bison.html

http://compilers.iecc.com/comparch/article/98-01-079

GPPG, the parser I’m using

Bison manual

Answer 1

Your revised ELSEIF rule has no markers for a condition — it should nominally have ‘(‘ and ‘)’ added.

More seriously, you now have a rule for

elsebody : else          | elseifs else          ; 

and

elseifs : /* Nothing */         | elseifs ...something...          ; 

The ‘nothing’ is not needed; it is implicitly taken care of by the ‘elsebody’ without the ‘elseifs’.

I would be very inclined to use rules ‘opt_elseifs’, ‘opt_else’, and ‘end’:

flow : '#' IF '(' ')' statements opt_elseifs opt_else end      ;  opt_elseifs : /* Nothing */             | opt_elseifs '#' ELSIF '(' ')' statements              ;  opt_else : /* Nothing */          | '#' ELSE statements          ;  end : '#' END     ; 

I’ve not run this through a parser generator, but I find this relatively easy to understand.