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[XText] about grammar ambiguity, left recursion [message #46205]

Fri, 29 May 2009 11:45

Mikai Yang

Messages: 149
Registered: July 2009

Senior Member

Hi,
for my DSL, I have developed its ecore metamodel, which is directly mapped to its abstract sytax. As we know, we do not consider ambiguity and
left recursion in the abstract syntax. XText grammar would deal with concrete syntax and unamibiguous parsing, so XText deals with such situlations as
ambiguity, left recursion and operation precedence?
Thanks.
Michael.

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Re: [XText] about grammar ambiguity, left recursion [message #46296 is a reply to message #46205]

Fri, 29 May 2009 12:15

Sven Efftinge

Messages: 1823
Registered: July 2009

Senior Member

It is not allowed to write left recursive grammars. You'll have to
"left-factor" them.
Operator precedence is also defined through the way you write the parser
rules. Ambiguity is not allowed.

If you want for example write a simple arithmetic calculator, rules
could look like so:

PlusOperation returns Expression:
MultiplyOperation ({BinaryOp.left=current} operator=('+'|'-')
right=MultiplyOperation)*;

MultiplyOperation returns Expression:
PrimaryExpression ({BinaryOp.left=current} operator=('*'|'/')
right=PrimaryExpression)*;

PrimaryExpression returns Expression:
NumberLiteral |
'(' PlusOperation ')';

NumberLiteral :
value=INT;

Regards,
Sven

Michael schrieb:
> Hi,
> for my DSL, I have developed its ecore metamodel, which is directly
> mapped to its abstract sytax. As we know, we do not consider ambiguity
> and left recursion in the abstract syntax. XText grammar would deal with
> concrete syntax and unamibiguous parsing, so XText deals with such
> situlations as
> ambiguity, left recursion and operation precedence?
> Thanks.
> Michael.

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Re: [XText] about grammar ambiguity, left recursion [message #369111 is a reply to message #46296]

Tue, 28 July 2009 16:45

Hauke Fuhrmann

Messages: 333
Registered: July 2009

Senior Member

Another question to the grammar below:

The example grammar for expressions works fine, but any complex
expression needs parantheses. Is it possible to write the grammar such
that you can omit the parantheses in the PrimaryExpression when
referencing the PlusOperation?
All my tries lead to recursive cycles.

Cheers,
Hauke

Sven Efftinge schrieb:
> If you want for example write a simple arithmetic calculator, rules
> could look like so:
>
> PlusOperation returns Expression:
> MultiplyOperation ({BinaryOp.left=current} operator=('+'|'-')
> right=MultiplyOperation)*;
>
> MultiplyOperation returns Expression:
> PrimaryExpression ({BinaryOp.left=current} operator=('*'|'/')
> right=PrimaryExpression)*;
>
> PrimaryExpression returns Expression:
> NumberLiteral |
> '(' PlusOperation ')';
>
> NumberLiteral :
> value=INT;

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