What is the problem in this yacc grammar?

Question

I have a yacc parser in which commands like "abc=123" (VAR=VAR), abc=[1 2 3] (VAR=value_series/string) and abc=[[123]] can be parsed.

I need to parse abc=[xyz=[1 2 3] jkl=[3 4 5]].This grammar is failing due to ambiguity between rule 2 (I guess, it couldn't differentiate between value_series and the new rule.

I have tried a case: VAR_NAME EQUAL quote_or_brace model EQUAL quote_or_brace value_series quote_or_brace net EQUAL quote_or_brace value_series quote_or_brace quote_or_brace

It didn't work.

series:
   | PLUS series
   {
   }
   | series VAR_NAME EQUAL VAR_NAME
   {
     delete [] $2;
     delete [] $4;
   }
   | series VAR_NAME EQUAL quote_or_brace value_series quote_or_brace
   {
     delete [] $2;
   }
   | series VAR_NAME EQUAL quote_or_brace quote_or_brace value_series quote_or_brace quote_or_brace
   {
     delete [] $2;
   }
   | error
   {
     setErrorMsg(string("error"));
     YYABORT;
   };

Your question title is a little misleading I think, as what you seem to want is a grammar without any ambiguity. As for one way to remove ambiguity is to try to make your rules simpler. For example, instead of three rules beginning with series VAR_NAME EQUAL why not make a single rule as series var_assignment (or similar). Then var_assignment: VAR_NAME EQUAL assignment_rhs_expression. And so on. — Some programmer dude
– Some programmer dude, Commented Sep 9, 2019 at 10:29
why cant I add another rule in the same series? why is it unable to parse the new rule? — goldcommode
– goldcommode, Commented Sep 9, 2019 at 11:41
what I am looking for is a way to parse - abc=[xyz=[1 2 3] jkl=[3 4 5]]. What rule should I write to make it parse this? — goldcommode
– goldcommode, Commented Sep 9, 2019 at 17:00

Some programmer dude · Accepted Answer · 2019-09-10 10:57:03Z

0

If it was me, I would probably write a set of rules similar to this:

assignment_list
  : assignment
  | assignment assignment_list
  ;

assignment
  : VAR_NAME '=' assignment_rhs
  ;

assignment_rhs
  : expression
  | '[' opt_expression_list ']'  /* allows var1 = [ ... ] */
  ;

opt_expression_list
  : /* empty */
  | expression_list
  ;

expression_list
  : expression
  | expression expression_list
  ;

expression
  : VAR_NAME   /* a variable name, as in var1 = var2 */
  | NUMBER     /* some kind of number, as in var1 = 123 */
  | STRING     /* quoted string, as in var1 = "foo" */
  | assignment /* allows nesting of assignments, as in var1 = [var2 = 4] */
               /* also allows things like var1 = var2 = 123 */
  ;

Note that this isn't tested in anyway, and that I'm not fully sure about the recursion from expression to assigment.

answered Sep 10, 2019 at 10:57

Some programmer dude

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