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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