I still stand by my earlier statement that reduce/reduce conflicts are
almost always bad. Yes, there may be certain situations where they
are harmless, but as a matter of practice, one should do everything
possible not to have grammars with reduce/reduce conflicts.
-Dave
On Mar 23, 2009, at 4:06 PM, Bruce Frederiksen wrote:
>
> David Beazley wrote:
>> Reduce/reduce conflicts are almost always bad---they mean that
>> certain
>> rules in your grammar will never be triggered.
> This isn't always true. Consider the following grammar:
>
> # foo.py
>
> from ply import yacc
>
> tokens = ('A', 'B', 'C')
>
> def p_grammar(p):
> '''
> rule1 : rule2 B
> | rule2 C
>
> rule2 : rule3 B
> | rule4
>
> rule3 : A
>
> rule4 : A
> '''
>
> yacc.yacc()
>
> This has a reduce/reduce conflict, but all rules are used:
>
> A B B => rule3 B B => rule2 B => rule1
> A C => rule4 C => rule2 C => rule1
>
>
> -bruce
>
> P.S. Should PLY 3.1 report unused rules? On this input, it outputs:
>
> WARNING: no p_error() function is defined
> Generating LALR tables
> WARNING: 1 reduce/reduce conflict
> WARNING: reduce/reduce conflict in state 1 resolved using rule
> (rule3 -> A)
> WARNING: rejected rule (rule4 -> A)
>
> The final line says that (rule4 -> A) is rejected, but (rule4 -> A) is
> still used in other situations (as noted in the second example input,
> above). Maybe the WARNING just means that the rule was rejected in
> this
> one state.
>
> But if that's the case, the following input never uses (rule5 -> A)
> under any circumstance:
>
> # foo.py
>
> from ply import yacc
>
> tokens = ('A', 'B', 'C')
>
> def p_grammar(p):
> '''
> rule1 : rule2 B
> | rule2 C
>
> rule2 : rule3 B
> | rule4
> | rule5
>
> rule3 : A
>
> rule4 : A
>
> rule5 : A
> '''
>
> yacc.yacc()
>
> And yet the WARNING message is the same:
>
> WARNING: no p_error() function is defined
> Generating LALR tables
> WARNING: 3 reduce/reduce conflicts
> WARNING: reduce/reduce conflict in state 1 resolved using rule
> (rule3 -> A)
> WARNING: rejected rule (rule4 -> A)
> WARNING: reduce/reduce conflict in state 1 resolved using rule
> (rule3 -> A)
> WARNING: rejected rule (rule5 -> A)
> WARNING: reduce/reduce conflict in state 1 resolved using rule
> (rule4 -> A)
> WARNING: rejected rule (rule5 -> A)
>
> And there is nothing is parser.out indicating the (rule5 -> A) is
> never
> used.
>
> >
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