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Parameterized parser rule [message #991177]

Mon, 17 December 2012 15:39

Pawel Pogorzelski

Messages: 40
Registered: July 2009

Member

I have a following grammar snippet. It describes a list item. Please note that the DSL is whitespace sensitive and TextLine ends with a line delimiter and may not contain '\t'.

UnorderedItem:
	'• ' TextLine
	('\t' TextLine)*;

What I'm not able to do is defining nesting. One approach is to define a second level list as shown below (pseudocode for simplicity):

UnorderedItem0:
	'• ' TextLine
	('\t' TextLine)*
	UnorderedItem1*;

	UnorderedItem1:
	'\t• ' TextLine
	('\t\t' TextLine)*
	UnorderedItem2*;

	UnorderedItem2:
	'\t\t• ' TextLine
	('\t\t\t' TextLine)*
	UnorderedItem3*;

But this is not scalable. What I need is actually something like a parametrized parser rule:

UnorderedItem (prefix):
	prefix '• ' TextLine
	(prefix '\t' TextLine)*
	UnorderedItem(prefix + '\t')*;

Is there anything like it in xtext? Or maybe there's some other solution that fits better.

Cheers,
Pawel

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Re: Parameterized parser rule [message #991298 is a reply to message #991177]

Tue, 18 December 2012 12:18

Andreas Brieg

Messages: 48
Registered: November 2012

Member

As far as I know, there aren't parameterized rules. But you don't really need them. Just have a look at arithmetic expressions:

(((3 + 1) + 2) + 4)

The rules for that normally look like (ignoring parenthesis, pseudo code):

expr := number|expr ' + ' number
number := <just a number>

So you are basically building a tree structure and the depth in the tree is the level of nesting. In your case the depth in the tree is the indentation of your item.

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Re: Parameterized parser rule [message #991313 is a reply to message #991177]

Tue, 18 December 2012 13:32

Alexander Nittka

Messages: 1193
Registered: July 2009

Senior Member

Hi,

you'll have to bind your own lexer implementation that creates indent and dedent tokens for creating the block structure. There are some blog posts and examples on the web.

Search for "xtext lexer indent" or "xtext whitespace".

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: Parameterized parser rule [message #991327 is a reply to message #991177]

Tue, 18 December 2012 14:26

Pawel Pogorzelski

Messages: 40
Registered: July 2009

Member

Guys, thanks for looking into this. I'll look into hooking up a custom lexer.

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