Hi Laurent,
yes I have, but the mode switching is supposed to happen mid-parsing. I
hope to avoid having to interrupt the parse, because picking up after a
subparse is going to be hard.
I was looking for a way to communicate a change of mode with the action
class, but:
a) I don't think there is a way to make the grammar parameter visible to
the action class. At least, I can't think of one.
b) the relation between rule X in the grammar and method X in the action
class is hiding within the engine.
c) loosely formulated grammar and action handling don't seem to share a
timeline.
Thanks,
Laurent Rosenfeld schreef op 2018-08-01 19:57:
Hi Theo,
have you considered using only one grammar but simply calling it with
two different actions classes as a parameter depending on the mode you
want to use?
2018-08-01 16:41 GMT+02:00 Theo van den Heuvel
<vdheu...@heuvelhlt.nl>:
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
--
Theo van den Heuvel
Van den Heuvel HLT Consultancy
