Skip to main content


Eclipse Community Forums
Forum Search:

Search      Help    Register    Login    Home
Home » Modeling » TMF (Xtext) » Modifying the default grammar
Modifying the default grammar [message #947013] Tue, 16 October 2012 18:29 Go to next message
Hallvard Traetteberg is currently offline Hallvard TraettebergFriend
Messages: 672
Registered: July 2009
Location: Trondheim, Norway
Senior Member
Hi,

I use xbase and want to support using a tick (') behind a variable
reference to refer to the variable's previous value. The idea (at the
semantic level) is to have a hidden set of variables suffixed by
__PREVIOUS__ that remember the previous state and translate a reference
with the tick to refer to the hidden one. E.g. the identifier "a" refers
to "a" and "a'" refers to "a__PREVIOUS__".

The translation works, but I have problems redefining the grammar to
support this, without also redefining the ID terminal.

What I tried was the following:

// remove the use of tick in the String terminal
terminal STRING: '"' ( '\\' ('b'|'t'|'n'|'f'|'r'|'u'|'"'|"'"|'\\') |
!('\\'|'"') )* '"'

// support trailing tick, as 'previous' value
PreviousValueID: ValidID "'"?;

// use PreviousValueID in XMemberFeatureCall
XMemberFeatureCall returns xbase::XExpression: ...

The idea is to limit the use of the tick-suffixed references to variable
references. However, I still got syntax errors when I use the tick. I
also need to redefine the ID terminal to allow a trailing tick, but this
will allow it almost everywhere, including in class names. I couldn't
find other usages of tick in the grammar than in the STRING terminal, so
shouldn't it be enough to redefine only STRING and not ID?

Hallvard
Re: Modifying the default grammar [message #947056 is a reply to message #947013] Tue, 16 October 2012 19:22 Go to previous messageGo to next message
Sebastian Zarnekow is currently offline Sebastian ZarnekowFriend
Messages: 3108
Registered: July 2009
Senior Member
Hi Hallvard,

what kind of syntax error do you get?
Did you override XFeatureCall, too?

Overriding the ID rule should not be necessary.

Regards,
Sebastian
--
Looking for professional support for Xtext, Xtend or Eclipse Modeling?
Go visit: http://xtext.itemis.com

Am 16.10.12 20:29, schrieb Hallvard Trætteberg:
> Hi,
>
> I use xbase and want to support using a tick (') behind a variable
> reference to refer to the variable's previous value. The idea (at the
> semantic level) is to have a hidden set of variables suffixed by
> __PREVIOUS__ that remember the previous state and translate a reference
> with the tick to refer to the hidden one. E.g. the identifier "a" refers
> to "a" and "a'" refers to "a__PREVIOUS__".
>
> The translation works, but I have problems redefining the grammar to
> support this, without also redefining the ID terminal.
>
> What I tried was the following:
>
> // remove the use of tick in the String terminal
> terminal STRING: '"' ( '\\' ('b'|'t'|'n'|'f'|'r'|'u'|'"'|"'"|'\\') |
> !('\\'|'"') )* '"'
>
> // support trailing tick, as 'previous' value
> PreviousValueID: ValidID "'"?;
>
> // use PreviousValueID in XMemberFeatureCall
> XMemberFeatureCall returns xbase::XExpression: ...
>
> The idea is to limit the use of the tick-suffixed references to variable
> references. However, I still got syntax errors when I use the tick. I
> also need to redefine the ID terminal to allow a trailing tick, but this
> will allow it almost everywhere, including in class names. I couldn't
> find other usages of tick in the grammar than in the STRING terminal, so
> shouldn't it be enough to redefine only STRING and not ID?
>
> Hallvard
Re: Modifying the default grammar [message #947084 is a reply to message #947056] Tue, 16 October 2012 20:00 Go to previous message
Hallvard Traetteberg is currently offline Hallvard TraettebergFriend
Messages: 672
Registered: July 2009
Location: Trondheim, Norway
Senior Member
Sebastian,

You are right, I discovered a bit after posting. The problem was I
didn't override XFeatureCall (the rule used when there is no dot, which
is the case for ordinary variables).

I ended up redefining only the STRING terminal and the IdOrSuper rule,
which is used by XFeatureCall. XFeatureCall itself needn't be redefined.

Here's the overrides:

// remove apostrophe variant
terminal STRING: '"' ( '\\' ('b'|'t'|'n'|'f'|'r'|'u'|'"'|"'"|'\\') |
!('\\'|'"') )* '"';

// support trailing apostrophe, as 'previous' value
IdOrSuper: (ValidID "'"?) | "'" | "super";

I also needed to inject a new value converter for the IdOrSuper rule, so
the apostrophe suffix is converted to __PREVIOUS__.

I get a warning stating that "Decision can match input such as "'\''"
using multiple alternatives: 1, 2", but it seems to work...

Grammar overrides are very powerful, but you really need to understand
the grammar you're overriding.

Hallvard

On 16.10.12 12.22, Sebastian Zarnekow wrote:
> Hi Hallvard,
>
> what kind of syntax error do you get?
> Did you override XFeatureCall, too?
>
> Overriding the ID rule should not be necessary.
>
> Regards,
> Sebastian
Previous Topic:How do I create Java & Python parsers for the same Xtext DSL
Next Topic:[xbase] Extension functions, inlining and arrays
Goto Forum:
  


Current Time: Thu Sep 24 13:00:24 GMT 2020

Powered by FUDForum. Page generated in 0.01487 seconds
.:: Contact :: Home ::.

Powered by: FUDforum 3.0.2.
Copyright ©2001-2010 FUDforum Bulletin Board Software

Back to the top