Re: [Grammatica-users] inherent ambiguity issue

[Oliver Gramberg]

Hi Ram,

I think the error message appears
because the second alternative for "ConditionTerm" start with
an "(", and the first derives (via "Value") a string
that can also start with "(" because of the second alternative
in "Factor".

One solution is to allow those parentheses
in only one place. Here's an example (grammar for a different language):


---------------------------------------------------------------------------------
_expression_ = xor_expression {'OR'
xor_expression} ;
xor_expression = and_expression {'XOR'
and_expression} ;
and_expression = comparison {('&'
| 'AND') comparison} ;
comparison = equ_expression {('='
| '<>') equ_expression} ;
equ_expression = add_expression {comparison_operator
add_expression} ;
comparison_operator = '<' 
        |
'>' 
        |
'<=' 
        |
'>=' ;
add_expression = term {add_operator
term} ;
add_operator = '+' 
        |
'-' ;
term = unary_expression {multiply_operator
unary_expression} ;
multiply_operator = '*' 
        |
'/' 
        |
'MOD' ;
unary_expression = {unary_operator}
primary_expression ;
unary_operator = '-'
        |
'+' 
        |
'NOT' ;
primary_expression = constant
        |
variable 
        |
'(' _expression_ ')' 
         
    ...
         
     ;
---------------------------------------------------------------------------------

It seems you are trying with your
grammar to force the condition of an if(...) to be a comparison. The above
approach would allow anything derivable from "_expression_" to
appear there, so you would have to check during semantic analysis that
your restriction is adhered to. On the other hand, if I'm not mistaken
standard C allows if(1)...

Regards
Oliver Gramberg

Ram Marzin <address@hidden>
Sent by: address@hidden

28.11.2010 18:45

Please respond to
address@hidden

To	address@hidden
cc
Subject	[Grammatica-users] inherent ambiguity issue

Hello,
My name is Ram and I use grammatica SW as a part of my
acadmic project in the university.
I use grammatica to help me develop c parser (an important
part of my project).
 
I get an inherent ambiguity error which I cannot solve,
The code is:
/* _expression_ */
_expression_ = Term [ExpressionRest] ;
ExpressionRest = "+" _expression_
                    
    | "-" _expression_ ;
Term = Factor [TermRest] ;
TermRest = "*" Term
                   | "/"
Term
                   | "%"
Term ;
Factor = Atom
                   | "("
_expression_ ")" ;
Atom = Number
                   | STRING_DATA
                   | VARIABLE_NAME
                   | FunctionCall
 
                   | Casting;
  
Number = INT_NUMBER | FLOAT_NUMBER;

/* Condition Statement */
Condition = ConditionTerm [ConditionRest] ;
ConditionRest = AND Condition
                    
  | OR Condition ;
ConditionTerm = ConditionAtom
                    
  | "(" Condition ")" ;      
ConditionAtom = Value ConditionOperator Value ;
ConditionOperator = "==" | "!=" | ">"
| "<" | "<=" | ">=";
Value = _expression_;
 
The error is: 
Error: in pt.grammar: lines 163-164:
    inherent ambiguity in production 'ConditionTerm' starting
with
    token "("
ConditionTerm = ConditionAtom
                    
      | "(" Condition ")" ;
 
I understand why it happens, there is unlimit look ahead
option but how can I solve it ?
 
Thanks,
Ram.
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