[DGD] parse_string() & removal of ()'s

[Felix A. Croes]

> This question is in regard to the parse_string() kfun and the omission of
> the ()'s from production rules.  Basically, it's mentioned that functions
> can be used to shape the returned parse tree, but I'm having an extremely
> hard time visualizing it. :)  I guess the best way to ask this question or
> address the issue is to present an example.

And I'll respond with one:

    whitespace = / /
    word = /[a-z]+/

    Thing: Noun
    Thing: SubAdjs Noun
    Noun: word
    Adjs: word Adjs
    Adjs: word
    SubAdjs: Adjs ? do_the_sub_thing

Now for different versions of do_the_sub_thing():

    mixed *do_the_sub_thing(mixed *adjs)
    {
	return ({ adjs });
    }

=> ({ ({ "big", "blue" }), "bottle" })

    mixed *do_the_sub_thing(mixed *adjs)
    {
	return adjs;
    }

=> ({ "big", "blue", "bottle" })

    mixed *do_the_sub_thing(mixed *adjs)
    {
	return ({ });
    }

=> ({ "bottle" })

    mixed *do_the_sub_thing(mixed *adjs)
    {
	return adjs - ({ "blue", "green" });
    }

=> ({ "big", "bottle" })

    mixed *do_the_sub_thing(mixed *adjs)
    {
	return 0;
    }

=> 0

Regards,
Dworkin