Eclipse Community Forums: TMF (Xtext)

Help

Home

Home » Modeling » TMF (Xtext) » Help with grammar(How best to write this grammar)

Show: Today's Messages :: Show Polls :: Message Navigator

Help with grammar [message #1742824]

Wed, 07 September 2016 22:42

Waqas Ilyas

Messages: 80
Registered: July 2009

Member

Problem #1

Lets say I have three rules a, b and c. And I want to allow any sequence of these rules, except that b and c should appear only once. How best to write such a grammar?

Here is what I came up with:

root:
  a+=a*
  b=b
  a+=a*
  c=c
  a+=a*
;
a: 'a';
b: 'b';
c: 'c';

Is there a better way to write root grammar? b and c still have to be in a strict order, which is not ideal.

Problem #2
Have a look at this grammar:

root:
    any+=any*
    x=x
    any+=any*
;

any:
    name=ID
    '{'
        any+=any*
    '}'
;

x:
    name='x' '{' y=y '}'
;

y:
    name='y' '{' z=z '}'
;

z:
    name='z' '{' any+=any* '}'
;

Using this grammar I expected to be able to write a language like below:

a {
    b {
        
    }
    
    c {
        y {
            
        }
    }
}

x {
    y {
        z {
            the_end {}
        }
    }
}

However, I get an error due to the node "y" appearing under "c". Why is that? Is it because now that "y" has been used as a terminal in one of the rules, it cannot appear anywhere else in the language?

How to fix this grammar?

Report message to a moderator

Re: Help with grammar [message #1742835 is a reply to message #1742824]

Thu, 08 September 2016 05:14

Karsten Thoms

Messages: 762
Registered: July 2009
Location: Dortmund, Germany

Senior Member

To #1:

root:
  a+=a*
  (
    (b=b a+=a* c=c)
    |
    (c=c a+=a* b=b)
  )
  a+=a*
;

a: 'a';
b: 'b';
c: 'c';

To#2:
It is not only that 'y' is a terminal rule, this could be solved by a datatype rule that allows 'y' as a valid identifier instead of using ID directly. The other problem is that any's ID could also be 'y' and there is no other chance to distinguish it in your grammar. Usually you could solve that with syntactic predicates, but not in your case.

Need professional support for Xtext, EMF, Eclipse IDE?
Go to: http://devhub.karakun.com
Twitter : @kthoms
Blog : www.karsten-thoms.de

Report message to a moderator

Re: Help with grammar [message #1743040 is a reply to message #1742835]

Fri, 09 September 2016 20:03

Waqas Ilyas

Messages: 80
Registered: July 2009

Member

Thanks a lot Karsten for the reply. It helps a lot.

For the second problem, I presume a better way would be to implement the special hierarchy of x/y/z through validators, instead of grammar rules. And the grammar should instead be simplified like:

root:
    any+=any+
;

any:
    name=ID
    '{'
        any+=any*
    '}'
;

Report message to a moderator

Previous Topic:	ParseHelper accepts illegal input
Next Topic:	Problems parsing ruby on rails files with xtext

Goto Forum:

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

]

Current Time: Thu Apr 25 06:56:36 GMT 2024

.:: Contact :: Home ::.

Breadcrumbs

Sign up to our Newsletter