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Forward propogate rules to deturmine correct rule [message #674950]

Sun, 29 May 2011 17:38

Richard Cookman

Messages: 7
Registered: May 2011

Junior Member

I have two rules in xtext both of an indetermined number of tokens. These rules are very similar and only differentiate themselved from each other by the character at the end of the rule. They appear in the same place in the program.

So in other words I want to do something similar to this

if it is rule 1
then process it as rule 1
else
it isn't rule 1 so process it as rule two

If there is nice way of doing this then that would be great

Sorry if this has been asked before.

[Updated on: Sun, 29 May 2011 17:38]

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Re: Forward propogate rules to deturmine correct rule [message #674961 is a reply to message #674950]

Sun, 29 May 2011 19:14

Alexander Nittka

Messages: 1193
Registered: July 2009

Senior Member

Hi,

first of all rules like this make work for the parser very hard (and inefficient) and the language design should be that these problems do not occur at all (a "keyword" determining which grammar rule to use should be placed at the beginning, not at the end).

Model: rules+=Rule;
Rule: RuleWithCommaAtEnd|RuleWithDotAtEnd;
RuleWithCommaAtEnd: entry+=ID* ',';
RuleWithDotAtEnd: entry+=ID* '.';

Alex

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Re: Forward propogate rules to deturmine correct rule [message #675236 is a reply to message #674961]

Mon, 30 May 2011 20:54

Richard Cookman

Messages: 7
Registered: May 2011

Junior Member

I had a feeling that might be the answer, sadly in this case it cannot be avoided.

I'm not sure but it might be the complexity of my situation but I get errors when the two rules are OR-ed together.

I don't know if it is because my rules uses recursion heavily, would that make a difference? Either way I don't see a way around this.

I have now changed my compiler implementation (not a big problem in these circumstances).

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