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Ambiguous rules and Predicate behavior [message #1861641]

Tue, 24 October 2023 23:25

Sohaib Hamioud

Messages: 4
Registered: September 2023

Junior Member

I am stuck with the following ambiguous rules in Xtext:

UnaryExpression returns ExpressionAtom:

..................

| EXISTS? '(' SelectStatement ')'

| '(' Expressions ')'
//nestedExpression
;

When I generate artifacts I get a warning about multiple alternatives. The '(' token is the source of ambiguity.

If I use a predicate "=>" to select the first alternative (i.e., the SelectStatement rule), I ended up with a never-reached alternative2. the SelectStatement rule starts with the 'select' keyword. I thought that if the selected alternative fails, the parser will automatically try the second alternative, but it seems that it is a one-way parsing. What should I do to force the parser to try other alternatives if the chosen one fails?

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Re: Ambiguous rules and Predicate behavior [message #1862547 is a reply to message #1861641]

Mon, 11 December 2023 12:17

Oscar Ablinger

Messages: 6
Registered: August 2023

Junior Member

The issue is that if EXISTS is not found, then a ( could both refer to the first as well as the second alternative.
If you want to make it LL(1) you could for instance write it like this:

| EXISTS '(' SelectStatement ')'
| '(' (Expressions | SelectStatement) ')

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