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Redundant usage of =current [message #1835352]

Tue, 01 December 2020 12:18

Ed Willink

Messages: 7655
Registered: July 2009

Senior Member

Hi

The Xtext grammar contains

AssignableAlternatives returns AbstractElement:
    AssignableTerminal ({Alternatives.elements+=current} ('|' elements+=AssignableTerminal)+)?
;

Is there any advantage to this obscure non-EBNF-like form compared to the regular

AssignableAlternatives returns AbstractElement:
    AssignableTerminal  
|   {Alternatives} elements+=AssignableTerminal ('|' elements+=AssignableTerminal)+
;

More generally is there any example of

... X ... {Y.f=current ...)? ...

that cannot be more readably rewritten as

... X ...  ...
| {Y} ... ... f=X ... ...

https://bugs.eclipse.org/bugs/show_bug.cgi?id=565093#c3 files this as an Xtext enhancement.

Regards

Ed Willink

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Re: Redundant usage of =current [message #1835353 is a reply to message #1835352]

Tue, 01 December 2020 12:29

Christian Dietrich

Messages: 14667
Registered: July 2009

Senior Member

as the

{Alternatives.elements+=current}

rewrites the ast/tree and

elements+=AssignableTerminal

does not, they do not do the same.

e.g. for mathematical expressions
https://www.eclipse.org/Xtext/documentation/307_special_languages.html#expressions

the rewrite will give you proper trees that care about precedences

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

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Re: Redundant usage of =current [message #1835368 is a reply to message #1835353]

Tue, 01 December 2020 19:44

Ed Willink

Messages: 7655
Registered: July 2009

Senior Member

Hi

Yes, but it only rewrites because it was parsed wrongly. By not using current, the correct AST is parsed directly. In both cases the net result is a single AssignableTerminal else an Alternatives with two or more AssignableTerminal elements .

Regards

Ed Willink

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Re: Redundant usage of =current [message #1835389 is a reply to message #1835368]

Wed, 02 December 2020 09:08

Ed Willink

Messages: 7655
Registered: July 2009

Senior Member

Hi

As pointed out by Sebastian on https://bugs.eclipse.org/bugs/show_bug.cgi?id=565093 , the real reason is that the rewrite is left-recursive and Xtext/ANTLR does not support left recursion. current= is an effective trick. It would be nice if it was hidden.

Regards

Ed Willink

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