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How to maka a rule LL(*) while preserving semantics? [message #1767318]

Tue, 04 July 2017 15:49

David Black

Messages: 33
Registered: June 2017

Member

Hi,

whenever I have a rule like this:

A -> cA | cAB | "a"
(..)

XText asks me for applying left-refactoring. On paper, I'd do it like this:

A -> cC | "a"
C -> A | AB
(...)

But in XText you don't have "null" rules, I believe. So I do it by mixing in regular expressions:

A -> cA (B)? | "a"

And XText seems to swallow it as a "left refactorization".

The problem is that in my parsed model I want cA and cAB to mean different things and to be mapped into different ECore classes, and as I understand, my "regular expression" impedes that.

Let's show it on a real XText snippet. How can I make the following rule LL(*) and at the same time get different and meaningful ECore classes for alternatives 2 and 3?

terminal TYPE:
	ID |
	('(' TYPE ')') |
	('(' TYPE ')' 'throws')
;

I've tried this hoping that it would guide XText throughout choices, but it did not work:

terminal TYPE:
	ID |
	COMPLICATED_TYPE
;

terminal fragment COMPLICATED_TYPE:
	('(' TYPE ')') |
	COMPLICATED_TYPE_WITH_THROWS
;

terminal fragment COMPLICATED_TYPE_WITH_THROWS:
	('(' TYPE ')' 'throws')
;

Cheers, David.

[Updated on: Tue, 04 July 2017 15:50]

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Re: How to maka a rule LL(*) while preserving semantics? [message #1767321 is a reply to message #1767318]

Tue, 04 July 2017 15:56

Christian Dietrich

Messages: 14716
Registered: July 2009

Senior Member

i cannot follow you.

terminals give you no eclasses at all.
so can you give us a real real world example to look at

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

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Re: How to maka a rule LL(*) while preserving semantics? [message #1767323 is a reply to message #1767321]

Tue, 04 July 2017 16:11

David Black

Messages: 33
Registered: June 2017

Member

OK, my mistake.

I wish to rewrite a rule so that

1) Get's converted into LL(*), because XText complains that the rules it's not LL(*)
2) Ensures that I will have different generated classes for these two parsed lines:

(mytype)

and

(mytype) throws

The rule I want to fix is this one, TYPE:

grammar org.xtext.example.mydsl.MyDsl with org.eclipse.xtext.common.Terminals

generate myDsl "http://www.xtext.org/example/mydsl/MyDsl"
Model:
	greetings+=Greeting*;
	
Greeting:
	name=TYPE;
	
terminal TYPE:
	ID |
	('(' TYPE ')') |
	('(' TYPE ')' 'throws')
;

[Updated on: Tue, 04 July 2017 16:14]

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Re: How to maka a rule LL(*) while preserving semantics? [message #1767324 is a reply to message #1767323]

Tue, 04 July 2017 16:18

Christian Dietrich

Messages: 14716
Registered: July 2009

Senior Member

but why terminals?

Type:
	{TypeWithId}name=ID | Rest
;

Rest returns Type:
	=>({TypeWithThrows}'(' type=Type ')' 'throws') |
	{TypeWithOutThrows}'(' type=Type ')' 
;

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

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Re: How to maka a rule LL(*) while preserving semantics? [message #1767327 is a reply to message #1767324]

Tue, 04 July 2017 16:28

David Black

Messages: 33
Registered: June 2017

Member

Christian Dietrich wrote on Tue, 04 July 2017 16:18

but why terminals?

Ok, they do not have to be terminals.

[Updated on: Tue, 04 July 2017 16:29]

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Re: How to maka a rule LL(*) while preserving semantics? [message #1767329 is a reply to message #1767324]

Tue, 04 July 2017 16:32

David Black

Messages: 33
Registered: June 2017

Member

Thanks for the solution!

Hmm, those are actions, aren't they? I'm afraid I still can't understand them well. I wasn't able to make much from the official documentation.

Anyway, that '=>' symbol... so that's the solution, right? I read in the official docs that it "tells" XText which option to choose on ambiguous cases. However I did not look at it as an alternative because I still don't understand how to use it in most of the cases.

Anyway, I will use it in this case ;)

Thanks again!

[Updated on: Tue, 04 July 2017 16:34]

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