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a problem in syntactic predicate [message #721197]

Thu, 01 September 2011 07:18

lulu

Messages: 19
Registered: August 2011

Junior Member

Hi, All,

There has a problem while practice syntactic predicate.

xtext grammar like this:
----- begin -------
a:
=> e ';'
| e '.'
;

e:
=> '(' ID ')' ID //type cast
| '(' ID ')' //nest expression
| ID
;

terminal ID: ('a'..'z')+;
----------- end ----------

while run as eclipse application, it occur some mistake in .dsl file.

But after I add brace in syntactic predicate. grammar show as follow:
----- begin -------
a:
=> ( e ';' )
| e '.'
;

e:
=> ( '(' ID ')' ID ) //type cast
| '(' ID ')' //nest expression
| ID
;

terminal ID: ('a'..'z')+;
----------- end ----------

It is run correctly which can match follow pattern:
test.
(test).
(test)test.
test;
(test);
(test)test;

I did not find any detail description in syntactic predicate about Xtext.

Thanks.

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Re: a problem in syntactic predicate [message #721202 is a reply to message #721197]

Thu, 01 September 2011 07:24

Christian Dietrich

Messages: 14669
Registered: July 2009

Senior Member

Hi,

what about http://www.eclipse.org/Xtext/documentation/2_0_0/020-grammar-language.php#antlr_errors

~Christian

Twitter : @chrdietrich
Blog : https://www.dietrich-it.de

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Re: a problem in syntactic predicate [message #721216 is a reply to message #721202]

Thu, 01 September 2011 07:54

lulu

Messages: 19
Registered: August 2011

Junior Member

Thanks to Christian.

I already read this part before practice it.

I dont know why this problem occurs?

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Re: a problem in syntactic predicate [message #721338 is a reply to message #721216]

Thu, 01 September 2011 14:05

David Wegener

Messages: 1445
Registered: July 2009

Senior Member

The documentation needs additional explanation about what a syntactic predicate is and how to use it. The current explanation is confusing. The grammar doesn't appear to match the language snippet. The grammar defines a 'then' key word but the sample code doesn't include the then. The explanation for choosing else basically says choose this 'else' not that 'else'. However, the grammar only has one else defined. Where does the other else come from and how does the syntactic predicate tell me which one to pick?

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Re: a problem in syntactic predicate [message #721345 is a reply to message #721338]

Thu, 01 September 2011 14:22

Alexander Nittka

Messages: 1193
Registered: July 2009

Senior Member

Hi,

the else is optional in the grammar, so the question is whether the else of the inner if or that of the outer if is to be matched. The syntactic predicate says, if you can match, do so immediately (and do not look for another branch of the tree, that could match as well). For that reason the inner if wins.

Alex

Need training, onsite consulting or any other kind of help for Xtext?
Go visit http://xtext.itemis.com or send a mail to xtext@itemis.de

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Re: a problem in syntactic predicate [message #721443 is a reply to message #721345]

Thu, 01 September 2011 19:08

David Wegener

Messages: 1445
Registered: July 2009

Senior Member

What would the grammar look like if you wanted to match the outer if instead of the inner if.

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Re: a problem in syntactic predicate [message #721521 is a reply to message #721338]

Fri, 02 September 2011 01:25

lulu

Messages: 19
Registered: August 2011

Junior Member

dpwegener wrote on Thu, 01 September 2011 10:05

The documentation needs additional explanation about what a syntactic predicate is and how to use it. The current explanation is confusing. The grammar doesn't appear to match the language snippet. The grammar defines a 'then' key word but the sample code doesn't include the then. The explanation for choosing else basically says choose this 'else' not that 'else'. However, the grammar only has one else defined. Where does the other else come from and how does the syntactic predicate tell me which one to pick?

I totally agree with this view.
I am confuse that whether the syntactic predicate is different from that in ANTLR?
And the grammar is from the ANTLR reference book.

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Re: a problem in syntactic predicate [message #721522 is a reply to message #721345]

Fri, 02 September 2011 01:30

lulu

Messages: 19
Registered: August 2011

Junior Member

Alexander Nittka wrote on Thu, 01 September 2011 10:22

Hi,

the else is optional in the grammar, so the question is whether the else of the inner if or that of the outer if is to be matched. The syntactic predicate says, if you can match, do so immediately (and do not look for another branch of the tree, that could match as well). For that reason the inner if wins.

Alex

But in xtext 2.0.0: syntactic predicate can use in ambiguous situation.

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Re: a problem in syntactic predicate [message #721564 is a reply to message #721443]

Fri, 02 September 2011 06:34

Alexander Nittka

Messages: 1193
Registered: July 2009

Senior Member

Quote:

What would the grammar look like if you wanted to match the outer if instead of the inner if.

I may be wrong, but I think that you cannot express that within the grammar. You would need to adapt the model instantiation yourself.

Alex

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Re: a problem in syntactic predicate [message #721650 is a reply to message #721564]

Fri, 02 September 2011 12:31

Sven Efftinge

Messages: 1823
Registered: July 2009

Senior Member

please file a bugzilla regarding the documentation improvements.

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Re: a problem in syntactic predicate [message #721750 is a reply to message #721564]

Fri, 02 September 2011 17:28

David Wegener

Messages: 1445
Registered: July 2009

Senior Member

On 09/02/2011 01:34 AM, Alexander Nittka wrote:
> Quote:
>> What would the grammar look like if you wanted to match the outer if
>> instead of the inner if.
>
>
> I may be wrong, but I think that you cannot express that within the
> grammar. You would need to adapt the model instantiation yourself.
>
> Alex
That would seem to indicate that the syntactic predicate isn't needed.
If the grammar can only be interpreted in one way, why would you need to
add the =>? This is part of the reason that syntactic predicate is so
confusing.

I'm not an expert on writing a language and am trying to learn more how
using Xtext etc. In looking through the XBase grammar, I've seen
syntactic predicates being used. Not having a good understanding of the
concept makes it difficult to figure out what the grammar rules are.

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Re: a problem in syntactic predicate [message #721881 is a reply to message #721750]

Sat, 03 September 2011 10:34

Alexander Nittka

Messages: 1193
Registered: July 2009

Senior Member

Hi,

I did say that the grammar cannot enforce the outer else to be matched. But without syntactic predicates you cannot enforce that the inner else is to be matched either (which you now can), so there is a big difference in having or not having them.

Without syntactic predicates a grammar could be ambiguous and in some cases you had to enable backtracking which is bad if you want to have a performant parser. Syntactic predicates now provide a way to guide the parser without having to enable backtracking. It is useful for many situations that could not be handled previously.

Alex

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Re: a problem in syntactic predicate [message #722457 is a reply to message #721197]

Mon, 05 September 2011 20:12

Sebastian Zarnekow

Messages: 3118
Registered: July 2009

Senior Member

Hi Lulu,

the reference documentation contains a section on syntactic predicates:
http://www.eclipse.org/Xtext/documentation/2_0_0/020-grammar-language.php#syntax

I'm afraid there is not more information available.
The predicate guards the element that is .. well .. predicated. That is,
if you use it for a group =>(e ';'), the parser will look for an e
followed by a ';', if you use it only for the =>e, the following ';'
will not be considered for the decision. Does that make sense to you?

Regards,
Sebastian
--
Need professional support for Eclipse Modeling?
Go visit: http://xtext.itemis.com

On 01.09.11 09:18, lulu wrote:
> Hi, All,
> There has a problem while practice syntactic predicate.
>
> xtext grammar like this:
> ----- begin -------
> a:
> => e ';' | e '.'
> ;
>
> e: => '(' ID ')' ID //type cast
> | '(' ID ')' //nest expression
> | ID
> ;
>
> terminal ID: ('a'..'z')+;
> ----------- end ----------
>
> while run as eclipse application, it occur some mistake in .dsl file.
>
>
> But after I add brace in syntactic predicate. grammar show as follow:
> ----- begin -------
> a:
> => ( e ';' )
> | e '.'
> ;
>
> e: => ( '(' ID ')' ID ) //type cast
> | '(' ID ')' //nest expression
> | ID
> ;
>
> terminal ID: ('a'..'z')+;
> ----------- end ----------
>
> It is run correctly which can match follow pattern:
> test.
> (test).
> (test)test.
> test;
> (test);
> (test)test;
>
> I did not find any detail description in syntactic predicate about Xtext.
>
> Thanks.
>

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