Have you considered subclassing your grammar? The child inherits the rules
from the parent, and an override the changed methods. Or, the parent
grammar could be rules-and-common-methods-only, and then have two child
grammas for "inside parens" vs "outside parens."
Not sure the mechanics of implementing that.
-y
On Wed, Aug 1, 2018 at 7:41 AM, Theo van den Heuvel <vdheu...@heuvelhlt.nl>
wrote:
>
>
> Hi Perl6-people,
>
> I am looking for some inspiration. I am working with a grammar that I
> would like to have operate in two different modes.
> In both modes the rules are identical, but the methods should behave
> differently. There are probably better ways to do this than I can think of
> at this point.
>
> As an illustration (not the actual problem), say we want to proces
> arithmetical expressions in two modes: normally we just copy the input, but
> within a pair of braces we directly calculate the result. grammar actions
> are easy to write per mode, but the combination is harder.
>
> In our example we would like to convert "{3 + 5} + {2 -1}" into "8 + 1".
> In my original case the grammar is large.
>
> So far, I have considered the following ideas:
> - using a parameter on the grammar rules
> - meddling with the AST
> - using a dynamic variable (but actions are performed later)
> - using the actions method (I don't see how I could use that here)
>
> One way to get this done is by combining the first two ideas:
>
> grammar Sum {
> token TOP { ^ <Sum: 0> $ }
> rule Sum($p) { <Expr>+ % <op> <flag>**{$p}}
> rule Expr { <num> | '[' ~ ']' <Sum: 1> }
> token op { <[-+]> }
> token num { \d+ }
> token flag { <?> }
> }
>
> The presence of the flag is the clue for the actions.
>
> This is less than satisfactory because we would have to pass on the
> parameter to all non-terminals.
>
> Can anyone think of a better way to do this?
>
> thanks,
>
> --
> Theo van den Heuvel
>
