Shift-reduce conflict between grammar rule and terminal in Xtext

Question

This is an example grammar:

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

generate myDsl "http://www.xtext.org/example/mydsl/MyDsl"

START: elem += DOG ;

DOG: 'DOG' INT ';' ;

terminal CAT : ('A'..'Z')('A'..'Z')('A'..'Z')' '('0'..'9')+;

When Xtext sees the line: DOG 1234, it can't resolve and will give the error "mismatch input DOG 1234... expecting DOG" something like that. I thought that when Xtext encounters a keyword, it should have higher precedence than the terminal. But it seems like it's confused. How can I fix this?

Note that DOG and CAT is used in different placed (i.e. the terminal CAT is used in other rule, but its definition conflicts with DOG rule)

Could you clarify your grammar just a bit. I am personally little bit confused with use of terminology. FOO is your production rule, there is 'FOO' again and then there is a terminal definition called BAR. Was 'FOO' supposed to be of type BAR? — Edmon
Well I fixed it. Hoped it's easier. Nevertheless, it's the same grammar rule in my actual grammar, just different terminologies. The rule DOG will recognize input strings like DOG 134, DOG 1234, but the terminal CAT will recognize the same thing also. — Amumu

Semih Korkmaz Semih Korkmaz · Accepted Answer · 2012-08-27T13:07:11

In this situation, as you can see, error is not "Expection CAT_RULE ..". So when you write DOG1234 as an element, Xtext is not parsing it as DOG and INT , instead it is getting 'DOG1234' as String, then looks for DOG rule and of course DOG rule only allows "DOG" keyword.

For example if you define DOG as another Terminal rule AFTER the CAT rule, also an element RAT 'conflicting' with CAT RULE and separate it's INT value from keyword by using new line you will see all of them works.

    START: elem += DOG 
    catelem += CAT 
    ratelem += RAT;

    RAT:
        "RAT"INT";"
    ;



terminal CAT : ('A'..'Z')('A'..'Z')('A'..'Z')' '('0'..'9')+;
terminal DOG : "DOG"("0".."9")+";";

And then usage of your new DSL example:

DOG1234;
CSA 1 
RAT
12347
;

Shift-reduce conflict between grammar rule and terminal in Xtext

1 Answers