Using named rules to key formatting sounds like a good idea; but I'm not sure that I like the idea of using a "derived grammar" to represent the formatting of these named rules: it seems beeter to introduce an explicit formatting object. This formatting object would provide information to a visitor that traverses a parse tree (i.e. a match object plus its grammar); but it would not have the hierarchical structure of the grammar.

Using a separate formating object has several advantages. Firstly, it avoids the parallel hierachies maintenance problem (you want formatting to be robust against modifications to the grammar). Even if you are only overriding the terminal rules of the grammar, you still have the issue that a change to the base grammar would be obscured in the derived grammar.

A more important issue is that it will be common to import terminal symbols from other grammars (e.g. CORE::*). How many grammars to you want to override, just to format some output?

I see a formatting object as a simple map of rule_name => format spec; with some mechanism for composition. This doesn't need to follow the hierarchy of a grammer: 

my $format = new Format (
  ws => { col >= 70 ? "\n" : " " },
  currency => { sprintf("%.2f", $currency) },
  date => {sprintf("%04d-%02d-%02d", @$date{qw/year month day/})},
);

$text =~ /<grammar.rule>/;

$format.output($0; grammar=>"grammar.rule", fh=>$*STDOUT)

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The output method would traverse the grammar; each time it meets a rule that has a format, it applies that format to the corresponding entry in the match object ($0). This does not require a derived grammar: just an extra specification object. Sometimes composition is better than inheritance.

--Dave

In reply to Re: Re: Re: Using Perl6 patterns/grammar definition for <I>output</I>? by dpuu
in thread Using Perl6 patterns/grammar definition for 'output'? by John M. Dlugosz

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