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problem with xtext and antlr [message #1133242]

Fri, 11 October 2013 14:14

Eclipse User

0 down vote favorite

I'm using xText for writing a new eclipse plugin for a new language. But when I try to generate the code Antlr give me this warning:

Decision can match input such as "'?'" using multiple alternatives: 1, 2

I'm sure, after commenting much code, that the problem is in the following code snip:

...
Expression:
     operatorExpr=OperatorExpr (condExpr=CondExpr)?
     |exprPrimary=ExprPrimary (condExpr=CondExpr)?
;

CondExpr:
     '?'
;


ExprPrimary:
     Identifier
;

OperatorExpr:
     '+' expression=Expression
;
...

How can i solve the warning?

Re: problem with xtext and antlr [message #1135539 is a reply to message #1133242]

Sun, 13 October 2013 03:56

Eclipse User

Given e.g.

+ <someIdentifier> ?

the parser cannot decide whether the '?' belongs to the Expression
holding the OperatorExpression or the one holding the PrimaryExpression.
This is equivalent to the famous dangling-else problem (Google).

You could use a syntactic predicate to solve this, but it looks like
you're trying to build an expression grammar, which should be structured
differently in LL grammars. I'd suggest to read
http://blog.efftinge.de/2010/08/parsing-expressions-with-xtext.html

Am 11.10.13 21:09, schrieb Luca Puglia:
> 0 down vote favorite
>
>
> I'm using xText for writing a new eclipse plugin for a new language. But
> when I try to generate the code Antlr give me this warning:
>
> Decision can match input such as "'?'" using multiple alternatives: 1, 2
>
>
>
> I'm sure, after commenting much code, that the problem is in the
> following code snip:
>
> ..
> Expression:
> operatorExpr=OperatorExpr (condExpr=CondExpr)?
> |exprPrimary=ExprPrimary (condExpr=CondExpr)?
> ;
>
> CondExpr:
> '?'
> ;
>
>
> ExprPrimary:
> Identifier
> ;
>
> OperatorExpr:
> '+' expression=Expression
> ;
> ..
>
> How can i solve the warning?
>

--
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Re: problem with xtext and antlr [message #1139572 is a reply to message #1135539]

Tue, 15 October 2013 17:38

Eclipse User

Can you please let me know how did you deploy the xtext project ? Is it integrated with eclipse as a plug-in?

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