The warning message about rules being rejected is new in PLY-3.0.
However, they are state dependent so I think I will clarify the
message to more clearly indicate the state. The idea of detecting a
rule that's never reduced is interesting. PLY-2 never reported any
errors about that nor does PLY-3. However, that seems like something
that would be pretty useful to add. I'll look into that for PLY-3.2
(working on it now).
Cheers,
Dave
On Mar 23, 2009, at 5:50 PM, Bruce Frederiksen wrote:
>
>
> But on a different topic, I was wondering whether you went over the
> material I had included in the P.S. of my message. Basically, I'm
> not sure whether you are thinking that PLY should report unreachable
> rules or not. If not, no problem. If so, then either the second
> example in the P.S. doesn't seem to produce any such notification;
> or the first example produces an erroneous notification (depending
> on what the notice looks like). I only bring this up because you
> say that PLY is used in a teaching setting and is designed to catch
> lots of errors...
>
> -bruce
>> 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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