Re: [Grammatica-users] token priority?

[Per Cederberg]

Thanks Iain! Great example and just the solution
I'd recommend.

Note to John: You can also check the calculator
example grammar for a similar solution.

Cheers,

/Per

Iain McGinniss wrote:
> Hi John,
>
> In order to implement unary expressions I used a grammar structured like 
> the following:
>
> AdditiveExpression = MultiplicativeExpression AdditiveExpressionTail* ;
> AdditiveExpressionTail = (ADD | SUB) MultiplicativeExpression ;
>
> MultiplicativeExpression = UnaryExpression MultiplicativeExpressionTail* ;
> MultiplicativeExpressionTail = (MUL | DIV) UnaryExpression ;
>
> UnaryExpression = SUB ConstantOrPostfixExpression
>    | ConstantOrPostfixExpression ;
>
> Which, for a simple expression like -4 + 5 gives a parse tree of
>
> AdditiveExpression
> |->MultiplicativeExpression
>   |->UnaryExpression
>      |->SUB
>      |->ConstantOrPostfixExpression (4)
> |->AdditiveExpressionTail
>   |->ADD
>   |->MultiplicativeExpression
>      |->UnaryExpression
>         |->ConstantOrPostfixExpression (5)
>
> It's been a while since I've touched this stuff but that's the gist of it.
>
> Cheers,
> Iain
>
> John Ledbetter wrote:
> > Hello,
> >
> > I am working with a grammar for a simple mathematical language, and I 
> > am having trouble implementing a unary minus sign.
> >
> > In flex/bison, I would do something like...
> >
> > %left PLUS MINUS
> > %nonassoc UNARY
> >
> > // ....
> > somerule : expr MINUS expr
> >              | MINUS expr %prec UNARY
> >
> >
> > What is the equivalent construct in grammatica?
> >
> > Thanks,
> > John.
>
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